docs: add CLAUDE.md for Claude Code guidance

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Usage is a spec and CLI for defining CLI tools using KDL format. It generates shell completions, markdown docs, and man pages from a single spec file. Think OpenAPI/Swagger for CLIs.
+
+## Build and Test Commands
+
+```bash
+# Build all packages
+cargo build --all
+
+# Run all tests
+cargo test --all --all-features
+
+# Run a single test
+cargo test -p usage-lib test_name
+cargo test -p usage-cli test_name
+
+# Update snapshots (uses cargo-insta)
+cargo insta test --accept
+
+# Lint and format
+cargo clippy --all --all-features -- -D warnings
+cargo fmt --all
+prettier -w .
+
+# Full CI check
+mise run ci
+
+# Render completions, docs, and assets
+mise run render
+```
+
+## Workspace Structure
+
+Three crates in a Cargo workspace:
+
+- **lib** (`usage-lib`): Core library containing spec parsing, CLI argument parsing, shell completion generation, and documentation generation
+- **cli** (`usage-cli`): Command-line tool that wraps the library
+- **clap_usage**: Generates usage specs from clap Command definitions
+
+## Architecture
+
+### Spec Model (`lib/src/spec/`)
+
+The spec model represents a CLI definition parsed from KDL:
+
+- `Spec` - Root struct containing name, version, commands, global completers
+- `SpecCommand` - A command/subcommand with args, flags, and nested subcommands
+- `SpecFlag` - A flag definition (`-v`, `--verbose`, `--config <path>`)
+- `SpecArg` - A positional argument (`<input>`, `[optional]`, `<files>...`)
+- `SpecComplete` - Custom completion definitions (shell commands to run)
+- `SpecMount` - Mount another spec at a subcommand path
+
+Specs can be:
+
+1. Parsed from `.usage.kdl` files
+2. Extracted from embedded `# USAGE:` comments in scripts
+3. Generated from clap Command definitions
+
+### Shell Completion Generation (`lib/src/complete/`)
+
+Each shell has its own module generating completion scripts:
+
+- `bash.rs` - Uses `complete` builtin
+- `zsh.rs` - Uses `compdef`
+- `fish.rs` - Uses `complete` command
+- `powershell.rs` - Uses `Register-ArgumentCompleter`
+
+Completions call back to `usage complete-word` at runtime for dynamic completions.
+
+### Argument Parsing (`lib/src/parse.rs`)
+
+The `parse()` function parses command-line arguments against a spec, returning:
+
+- Matched command path
+- Parsed args and flags with values
+- Env var and default fallbacks applied
+
+### Documentation Generation (`lib/src/docs/`)
+
+Generates from specs:
+
+- Markdown documentation (`markdown/`)
+- Man pages (`manpage/`)
+- CLI help text (`cli/`)
+
+Uses Tera templates for markdown rendering.
+
+## KDL Spec Format
+
+Specs use KDL syntax. Key nodes:
+
+```kdl
+name "mycli"
+bin "mycli"
+flag "-v --verbose" help="Enable verbose output"
+arg "<input>" help="Input file"
+cmd "subcommand" {
+    flag "--force"
+    arg "[optional]"
+}
+complete "input" run="find . -name '*.txt'"
+```
+
+## Testing
+
+- Snapshot tests use `cargo-insta` with auto-review enabled
+- Shell integration tests require bash, zsh, fish, and pwsh installed
+- Run `cargo insta test --accept` to update snapshots
+
+## Key Dependencies
+
+- `kdl`: KDL parser for spec files
+- `clap`: CLI parsing for the usage tool itself
+- `miette`: Error reporting with diagnostics
+- `tera`: Template engine for markdown docs
+- `insta`: Snapshot testing