feat: Gemini CLI integration — MCP server config + lifecycle hooks

[buildingjoshbetter]

Closes #233

Summary

• Adds GeminiAdapter in truememory/hooks/adapters/gemini.py — single-file JSON adapter for ~/.gemini/settings.json with MCP server registration and lifecycle hooks (SessionStart, SessionEnd, PreCompress)
• 20 new tests covering import, detection, MCP install, hook install, idempotency, additive merge, uninstall, interface compliance, and system prompt
• Quickstart guide at docs/setup-gemini.md, compatibility matrix updated with Gemini column

Test plan

• All 20 new tests pass
• All 55 existing adapter tests still pass (zero regressions)
• uvx ruff check passes — no lint errors
• Import chain verified — no circular imports
• Interface compliance: all 11 CLIAdapter abstract methods implemented
• Idempotent install: running twice produces identical config
• Uninstall removes only TrueMemory entries, preserves other config
• Standalone hooks (python -m truememory.ingest.hooks.session_start) still importable
• All 6 existing adapters (Claude, Codex, Kimi, Hermes, OpenClaw) still load in registry