feat: GitHub Copilot package

[nathanschram]

Phase 4: GitHub Copilot Package (v1.9.0)

Context

GitHub Copilot now has a full plugin model with 5 primitives — agents, skills, hooks, MCP servers, and instructions. With 8 hook events (preview), skills following the open Agent Skills standard, and custom agents, Copilot can deliver a strong ContextDocs experience.

Platform Capabilities (GitHub Copilot)

• Hooks: 8 events (Preview) — sessionStart, sessionEnd, userPromptSubmitted, preToolUse, postToolUse, agentStop, subagentStop, errorOccurred
• Skills: .github/skills/<name>/SKILL.md — open Agent Skills standard
• Agents: .github/agents/*.agent.md — supports handoffs and workflow chaining
• Instructions: .github/instructions/*.instructions.md — path-scoped with applyTo globs
• Install: /plugin install owner/repo

Deliverables

1. platforms/copilot/.github/skills/ai-context/SKILL.md
2. platforms/copilot/.github/skills/context-verify/SKILL.md
3. platforms/copilot/.github/agents/context-updater.agent.md
4. platforms/copilot/.github/hooks/ — Port hooks (8 events)
5. platforms/copilot/.github/instructions/context-quality.instructions.md

Verification

• Skills activate in Copilot CLI and VS Code
• Agent runs context update workflow
• Hook events fire correctly

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