Add support for Windsurf Co-Authored-By detection

[arunroyjhs]

Summary

GitIntel currently detects Co-Authored-By trailers from Claude Code, Cursor, and Copilot. Windsurf is gaining traction and uses its own co-author format.

What needs to happen

1. Research the exact Co-Authored-By trailer format Windsurf uses in commits
2. Add the pattern to the agent detection logic in packages/cli/src/
3. Add test cases with sample Windsurf commits
4. Update the README agent support table

Why this matters

Windsurf is one of the fastest-growing AI coding tools. Supporting it makes GitIntel more comprehensive and discoverable by Windsurf users.

Good first issue because

• Self-contained change (pattern matching addition)
• Clear acceptance criteria
• Easy to test locally with a crafted commit