docs: add multi-agent support architecture, expand documentation

- Add AGENT_SUPPORT.md with comprehensive multi-agent architecture design
  - Agent capability model for controlling UI feature availability
  - Refactoring plan for abstracting Claude-specific code
  - Implementation guides for adding new agents (OpenCode, Gemini CLI, etc.)
  - Test impact analysis and migration strategy

- Expand ARCHITECTURE.md with new sections:
  - Achievement system, AI tab system, execution queue
  - Navigation history, web/mobile interface, CLI tool
  - Shared module, remote access & tunnels
  - Updated hooks documentation (15 custom hooks)
  - Expanded IPC API surface reference

- Update CONTRIBUTING.md:
  - Add detailed "Adding a New AI Agent" section
  - Agent capability checklist for new implementations
  - UI feature availability matrix

- Update CLAUDE.md with latest vernacular and patterns

- Various UI improvements:
  - Enhanced history panel with better modal handling
  - Session list refinements
  - Improved template variable handling
  - Mobile interface view state utilities

Claude ID: c19f7b47-0dca-46a5-b8b4-4ca2381c1c71
Maestro ID: b9bc0d08-5be2-4fdf-93cd-5618a8d53b35

.gitignore

@@ -1,4 +1,5 @@
 # Tests
+coverage/
 do-wishlist.sh
 do-housekeeping.sh
 coverage/