Add kubeconfig-style configuration file support

CONFIG.md

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+# Configuration File
+
+cortextool supports a kubeconfig-style configuration file for managing multiple Cortex clusters with ease. This allows you to define multiple clusters, authentication credentials, and contexts, and easily switch between them.
+
+## Quick Start
+
+1. **Create a config file** at `~/.cortextool/config`:
+
+```yaml
+current-context: production
+
+contexts:
+  - name: production
+    context:
+      cluster: prod-cortex
+      user: prod-user
+
+clusters:
+  - name: prod-cortex
+    cluster:
+      address: https://cortex.prod.example.com
+      tls-ca-path: /path/to/ca.crt
+      tls-cert-path: /path/to/client.crt
+      tls-key-path: /path/to/client.key
+
+users:
+  - name: prod-user
+    user:
+      id: tenant-123
+      auth-token: your-jwt-token
+```
+
+2. **Use cortextool commands** without specifying connection details:
+
+```bash
+# The address, tenant ID, and TLS certs are loaded from the config file
+cortextool rules list
+cortextool alertmanager get
+```
+
+3. **Switch between contexts**:
+
+```bash
+cortextool config use-context staging
+```
+
+## Configuration Structure
+
+The configuration file consists of four main sections:
+
+### 1. Current Context
+
+Specifies which context to use by default:
+
+```yaml
+current-context: production
+```
+
+### 2. Contexts
+
+Contexts bind together a cluster and user credentials:
+
+```yaml
+contexts:
+  - name: production
+    context:
+      cluster: prod-cortex    # references a cluster name
+      user: prod-user         # references a user name
+  - name: staging
+    context:
+      cluster: staging-cortex
+      user: staging-user
+```
+
+### 3. Clusters
+
+Clusters define connection details for Cortex instances:
+
+```yaml
+clusters:
+  - name: prod-cortex
+    cluster:
+      address: https://cortex.prod.example.com
+      tls-ca-path: /path/to/ca.crt          # Optional: TLS CA certificate
+      tls-cert-path: /path/to/client.crt    # Optional: Client certificate for mTLS
+      tls-key-path: /path/to/client.key     # Optional: Client certificate key
+      use-legacy-routes: false               # Optional: Use /api/prom/ routes
+      ruler-api-path: /api/v1/rules         # Optional: Custom ruler API path
+```
+
+### 4. Users
+
+Users define authentication credentials:
+
+```yaml
+users:
+  - name: prod-user
+    user:
+      id: tenant-123              # Tenant ID
+      auth-token: jwt-token-here  # Bearer token for JWT auth
+      # OR use basic auth:
+      # user: username
+      # key: password
+```
+
+## Configuration Precedence
+
+Configuration values are resolved in the following order (highest priority first):
+
+1. **Command-line flags**: `--address`, `--id`, `--tls-cert-path`, etc.
+2. **Environment variables**: `CORTEX_ADDRESS`, `CORTEX_TENANT_ID`, `CORTEX_TLS_CLIENT_CERT`, etc.
+3. **`--context` flag**: Override which context to use (instead of `current-context`)
+4. **Config file**: Values from the current context
+5. **Defaults**: Built-in defaults
+
+### Examples
+
+```bash
+# Use config file defaults (current-context)
+cortextool rules list
+
+# Override address with flag (other values still from config)
+cortextool rules list --address https://different-cortex.com
+
+# Override with environment variable
+export CORTEX_TENANT_ID=different-tenant
+cortextool rules list
+
+# Use a different context temporarily with --context flag
+cortextool --context staging rules list
+
+# Combine --context with individual flags
+cortextool --context production --id different-tenant rules list
+
+# Use a different config file
+cortextool --config /path/to/custom-config rules list
+```
+
+### Using the `--context` Flag
+
+The `--context` flag allows you to temporarily use a different context without changing the `current-context` in your config file. This is useful for:
+
+- **Quickly switching environments**: `cortextool --context staging rules list`
+- **Testing with different credentials**: `cortextool --context test-user alertmanager get`
+- **Scripts that need specific contexts**: Always use a specific context regardless of current-context
+
+**Example workflow:**
+
+```bash
+# Set up multiple contexts
+cortextool config set-context prod --cluster prod-cluster --user prod-user
+cortextool config set-context staging --cluster staging-cluster --user staging-user
+cortextool config use-context prod  # Set prod as default
+
+# Use prod (current-context)
+cortextool rules list
+
+# Temporarily use staging without changing current-context
+cortextool --context staging rules list
+
+# Still using prod by default
+cortextool rules list
+```
+
+**Error handling:**
+
+If you specify a context that doesn't exist, cortextool will fail with a clear error:
+
+```bash
+cortextool --context nonexistent rules list
+# Error: context "nonexistent" not found in config file
+```
+
+## Config Management Commands
+
+cortextool provides subcommands to manage your configuration file:
+
+### View Configuration
+
+Display the current configuration:
+
+```bash
+cortextool config view
+```
+
+### List Contexts
+
+Show all available contexts:
+
+```bash
+cortextool config get-contexts
+```
+
+Output:
+```
+CURRENT   NAME
+*         production
+          staging
+          development
+```
+
+### Show Current Context
+
+Display which context is currently active:
+
+```bash
+cortextool config current-context
+```
+
+### Switch Context
+
+Change the active context:
+
+```bash
+cortextool config use-context staging
+```
+
+### Manage Contexts
+
+Create or update a context:
+
+```bash
+# Create a new context
+cortextool config set-context my-context --cluster my-cluster --user my-user
+
+# Update an existing context
+cortextool config set-context production --cluster new-prod-cluster
+```
+
+### Manage Clusters
+
+Create or update cluster configuration:
+
+```bash
+cortextool config set-cluster prod-cortex \
+  --address https://cortex.prod.example.com \
+  --tls-ca-path /path/to/ca.crt \
+  --tls-cert-path /path/to/client.crt \
+  --tls-key-path /path/to/client.key
+```
+
+### Manage Credentials
+
+Create or update user credentials:
+
+```bash
+# JWT authentication
+cortextool config set-credentials prod-user \
+  --id tenant-123 \
+  --auth-token eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+
+# Basic authentication
+cortextool config set-credentials staging-user \
+  --id tenant-456 \
+  --user admin \
+  --key password123
+```
+
+### Delete Context
+
+Remove a context:
+
+```bash
+cortextool config delete-context old-context
+```
+
+## Default Config Location
+
+By default, cortextool looks for the config file at:
+
+```
+$HOME/.cortextool/config
+```
+
+You can override this with:
+
+- The `--config` flag: `cortextool --config /custom/path rules list`
+- Set a custom default when creating the config file
+
+## Example Workflows
+
+### Multi-Cluster Setup
+
+```bash
+# Set up production cluster
+cortextool config set-cluster prod --address https://cortex.prod.example.com
+cortextool config set-credentials prod-user --id prod-tenant --auth-token <token>
+cortextool config set-context prod --cluster prod --user prod-user
+
+# Set up staging cluster
+cortextool config set-cluster staging --address https://cortex.staging.example.com
+cortextool config set-credentials staging-user --id staging-tenant --auth-token <token>
+cortextool config set-context staging --cluster staging --user staging-user
+
+# Use production
+cortextool config use-context prod
+cortextool rules list
+
+# Switch to staging
+cortextool config use-context staging
+cortextool rules list
+```
+
+### Local Development
+
+```bash
+# Set up local development environment
+cortextool config set-cluster local --address http://localhost:9009
+cortextool config set-credentials dev --id dev
+cortextool config set-context dev --cluster local --user dev
+cortextool config use-context dev
+
+# Now work with local Cortex
+cortextool rules load rules.yaml
+```
+
+### Using TLS with mTLS
+
+```bash
+cortextool config set-cluster secure-cluster \
+  --address https://cortex.secure.example.com \
+  --tls-ca-path ~/.cortextool/certs/ca.crt \
+  --tls-cert-path ~/.cortextool/certs/client.crt \
+  --tls-key-path ~/.cortextool/certs/client.key
+
+cortextool config set-credentials secure-user --id tenant-secure --auth-token <token>
+cortextool config set-context secure --cluster secure-cluster --user secure-user
+cortextool config use-context secure
+```
+
+## Migration from Environment Variables
+
+If you're currently using environment variables, you can continue to use them alongside the config file. They will override config file values when set.
+
+To migrate, convert your environment variables to a config file:
+
+```bash
+# Before (environment variables)
+export CORTEX_ADDRESS=https://cortex.example.com
+export CORTEX_TENANT_ID=my-tenant
+export CORTEX_AUTH_TOKEN=my-token
+cortextool rules list
+
+# After (config file)
+cortextool config set-cluster my-cluster --address https://cortex.example.com
+cortextool config set-credentials my-user --id my-tenant --auth-token my-token
+cortextool config set-context default --cluster my-cluster --user my-user
+cortextool config use-context default
+cortextool rules list  # No environment variables needed!
+```
+
+## Troubleshooting
+
+### Config file not found
+
+If cortextool can't find your config file, ensure it exists at `~/.cortextool/config` or specify it with `--config`.
+
+### Invalid current context
+
+If you get an error about the current context:
+
+```bash
+# Check available contexts
+cortextool config get-contexts
+
+# Switch to a valid context
+cortextool config use-context <valid-context-name>
+```
+
+### Missing required fields
+
+If a command fails with "cortex address is required" or "tenant ID is required", ensure your context references valid cluster and user entries with all required fields set.
+
+## See Also
+
+- [cortextool.example.yaml](./cortextool.example.yaml) - Full example configuration file
+- [README.md](./README.md) - Main documentation