docs(manual): Add comprehensive Gavel CLI manual documentation

Introduce MANUAL.md with complete documentation of the Gavel CLI tool, covering:

- CLI tree structure and command hierarchy
- Shared operating model including config files, UI modes, and TODO flows
- Detailed .gavel.yaml configuration reference with merge rules
- Testing and quality commands (test, lint, fixtures, bench, summary, ui serve)
- Review and authoring commands (verify, commit, todos)
- PR, git history, and repository mapping commands
- Infrastructure and long-running service commands (ssh, system)
- Utility commands (completion, version)
- Common end-to-end workflows and command selection guide

This manual serves as the primary reference for users to understand Gavel's capabilities, command structure, and recommended workflows.

Author: moshloop