Add clash check command with Claude Code hook support

[mathew55]

Closes #2, closes #5

Summary

• Adds clash check command for single-file conflict detection across worktrees
• Supports two modes: manual (clash check <path>) with JSON on stdout, and hook mode (clash check with stdin) for Claude Code's PreToolUse hook
• In hook mode, reads tool_input.file_path from stdin JSON, outputs conflicts to stderr on exit 2 (which Claude sees), and is silent on exit 0
• Includes TTY guard so running bare clash check in a terminal shows helpful usage instead of blocking
• Updates WorktreeManager with find_containing() and iter() methods
• Adds conflicts_with() error field to WorktreePairConflict for surfacing detection failures

Hook configuration

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Write|Edit",
        "hooks": [
          {
            "type": "command",
            "command": "clash check"
          }
        ]
      }
    ]
  }
}

Test plan

• cargo build && cargo clippy -- -D warnings && cargo fmt --check
• Manual mode: clash check src/main.rs outputs JSON to stdout
• Hook mode: echo '{"tool_input":{"file_path":"src/main.rs"}}' | clash check reads from stdin
• TTY guard: bare clash check in terminal prints helpful error, doesn't block
• End-to-end: hook fires on Edit tool call and blocks conflicting file edits

🤖 Generated with Claude Code