fix: reload config changes without window restart

[hellodk]

Summary

Closes #129

Edits to ~/.champ/config.yaml (from a terminal or external tool) were not picked up until a VS Code window restart. The previous watcher relied on vscode.workspace.createFileSystemWatcher rooted at os.homedir() — out-of-workspace watching is unreliable on Linux, so external edits were frequently missed.

Changes

• New src/config/config-watcher.ts — reliable, debounced watcher combining:
  • Native FileSystemWatcher fast path (in-VS-Code edits) → signalChanged()
  • An mtime poller fallback (platform-independent, catches terminal/external edits)
  • Both debounce into a single loadProvider() call
• src/extension.ts rewritten watcher block: ConfigWatcher + native watcher feed one debounced reload path
• test/unit/config/config-watcher.test.ts — 5 tests (baseline, debounced single reload, collapse rapid changes, no reload on unchanged, deleted-file reload)

Tests required

• Baseline poll fires without invoking onChange
• mtime change fires onChange exactly once (debounced)
• Rapid changes collapse into a single reload
• No reload when mtime unchanged
• Deleted file triggers reload once

Gates: pnpm run check-types, pnpm vitest run (1751 pass), pnpm run lint (0 errors)