feat(copilot): add rtk init --agent copilot support

[jeziellopes]

Summary

Closes the gap left by #605 / #704: rtk hook copilot was shipped but rtk init had no Copilot counterpart. This PR adds rtk init --agent copilot.

Changes

File	Change
src/main.rs	Added Copilot variant to AgentTarget enum; routed install_copilot and uninstall
src/init.rs	Added run_copilot_mode(), COPILOT_INSTRUCTIONS const, uninstall support, install_copilot param
CHANGELOG.md	[Unreleased] entry

Behavior

# Install (project-scoped, no --global needed)
rtk init --agent copilot
# → writes .github/copilot-instructions.md
# → reports .github/hooks/rtk-rewrite.json status

# Idempotent — re-running is safe
rtk init --agent copilot
# → skips if RTK already detected in the file

# Uninstall
rtk init --uninstall --agent copilot
# → removes .github/copilot-instructions.md
# → leaves .github/hooks/rtk-rewrite.json untouched (may be shared)

Pattern followed

Mirrors Cline/Windsurf exactly:

• fn run_copilot_mode(verbose: u8) -> Result<()>
• Idempotent check on the target file
• Routed as a solo --agent target (short-circuits before the Claude/OpenCode match block)

Testing

979 passed; 0 failed; 3 ignored
cargo fmt --all --check: ok
cargo clippy --all-targets -- -D warnings: 0 warnings

Manually tested in a temp dir: install, idempotent re-run, and uninstall all work correctly.

Context

• PR feat(copilot): cross-platform PreToolUse hook via rtk hook (VS Code + Copilot CLI + Windows) #605 added the rtk hook copilot Rust subcommand (VS Code + Copilot CLI hook)
• PR feat: 9-tool AI agent support + emoji removal #704 merged it into master as part of the 9-agent release (v0.31.0)
• This PR completes Copilot support so users can rtk init --agent copilot like they can for Cursor, Windsurf, and Cline