feat: Gemini CLI extension package

[nathanschram]

Phase 3: Gemini CLI Extension (v1.8.0)

Context

Gemini CLI has the most complete extension model of any non-Claude-Code tool — 11 hook events, skills, agents, policies, and a full gemini-extension.json manifest. It's the ideal first platform to target for near-parity with the Claude Code experience.

Platform Capabilities (Gemini CLI)

• Hooks: 11 events — SessionStart, SessionEnd, BeforeAgent, AfterAgent, BeforeModel, AfterModel, BeforeToolSelection, BeforeTool, AfterTool, PreCompress, Notification
• Skills: Via skills/ directory in extensions
• Agents: Experimental, agent definitions as .md files
• Policies: Via .toml files in policies/ directory
• MCP servers: Yes, via extension manifest
• Install: npm install or local directory

Deliverables

1. platforms/gemini/gemini-extension.json — Extension manifest
2. platforms/gemini/skills/ai-context/SKILL.md — Adapted skill
3. platforms/gemini/skills/context-verify/SKILL.md — Adapted skill
4. platforms/gemini/hooks/ — Port session-start, post-commit drift, pre-commit guard hooks (adapt BeforeTool/AfterTool event names, Gemini JSON schema)
5. platforms/gemini/agents/context-updater.md — Adapted agent
6. platforms/gemini/policies/context-quality.toml — Quality rules as Gemini policies

Verification

• Extension installs successfully on Gemini CLI
• Skills activate on relevant prompts
• Hooks fire on correct events
• Agent runs context update workflow

Part of the Cross-Platform ContextDocs Strategy. Depends on #18.