Add --kubeconfig-dir flag for multi-kubeconfig support

[nadaverell]

Summary

Adds --kubeconfig-dir flag to support loading multiple kubeconfig files from directories. This enables easier migration from Lens, which stores configs as separate files rather than in a single merged kubeconfig.

Usage:

# Single directory
radar --kubeconfig-dir ~/.kube/configs/

# Multiple directories
radar --kubeconfig-dir ~/.kube/lens/,~/.kube/rancher/

Key behaviors:

• Scans directories for valid kubeconfig files
• Skips hidden files (.foo) and invalid files
• Merges configs using kubectl's standard Precedence mechanism
• All contexts from all files appear in the context switcher
• Context switching works seamlessly across files

Closes #30

[Cenness]

Did you test it? It crashes immediately on dir with invalid config

apiVersion: v1
kind: Config
clusters:
  - name: dr-prod-cluster
    cluster:
      server: https://10.0.0.0:6443
      certificate-authority-data: ""
users:
  - name: user
    user:
      client-certificate-data: ""
      client-key-data: ""
contexts: []
preferences: {}
current-context: cluster

[nadaverell]

Did you test it? It crashes immediately on dir with invalid config

apiVersion: v1
kind: Config
clusters:
  - name: dr-prod-cluster
    cluster:
      server: https://10.0.0.0:6443
      certificate-authority-data: ""
users:
  - name: user
    user:
      client-certificate-data: ""
      client-key-data: ""
contexts: []
preferences: {}
current-context: cluster

Actually did test on invalid config. Maybe invalid in a different way. Let me take another look.