Add project docs, tool manuals, and initial user tools ...

- CLAUDE.md: comprehensive project reference for Claude Code sessions
- manuals/write_tool.md: full format reference with working example
- tools/_template.py: canonical module-level tool format
- tools/enabled.json: auto-include list (get_current_datetime, user_notes)
- get_current_datetime, user_notes: first Navi-written tools

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