v0.5.0: chore: bump version to 0.5.0, add CHANGELOG, update docs

- Bump version: 0.4.1 → 0.5.0
- Add CHANGELOG.md with v0.5.0 entry covering notes support,
  dual-mode CLI/MCP architecture, and all changes since v0.4.0
- Update README version reference
- Update pyproject.toml description to mention CLI mode

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>