v0.3.9 — fix ctx-install (settings_patcher + plugin structure)

fix: settings_patcher + plugin structure + bump 0.3.8 → 0.3.9

- Add src/cli/settings_patcher.py (was missing from wheel — broke ctx-install
  on fresh pip install with ModuleNotFoundError)
- Move plugin.json to .claude-plugin/ root (was nested under plugin/.claude-plugin/)
- Bump version to 0.3.9

ctx-install dry-run verified: hook copy + settings patch both functional.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>