feat: rtk init --agent copilot (automated Copilot VS Code + CLI setup)

[pszymkowiak]

Context

The hook rtk hook copilot is implemented and works for both VS Code Copilot Chat and Copilot CLI. However, there is no automated setup — users must manually create the required files.

The README documents rtk init -g as supporting Copilot, but in reality it only installs the Claude Code hook. AgentTarget enum has Claude, Cursor, Windsurf, Cline — but not Copilot.

Proposal

Add rtk init -g --agent copilot that:

1. Creates .github/hooks/rtk-rewrite.json — PreToolUse hook config pointing to rtk hook copilot
2. Creates/patches .github/copilot-instructions.md — prompt-level RTK awareness
3. Verifies rtk binary is in PATH

Files to modify

• src/main.rs — add Copilot to AgentTarget enum
• src/init.rs — add run_copilot_mode() function
• hooks/copilot-rtk-awareness.md — template for instructions (already exists)

References

• Hook implementation: src/hook_cmd.rs (run_copilot, detect_format)
• VS Code hooks spec: https://code.visualstudio.com/docs/copilot/copilot-extensibility-overview
• Existing patterns: run_cursor_mode(), run_windsurf_mode(), run_cline_mode() in src/init.rs