docs: add CONTRIBUTING.md with team conventions

[MichelleGel]

Closes

Closes #5

Summary

Adds CONTRIBUTING.md documenting the team's working conventions, consolidated from the project plan:

• Branching model (GitHub Flow) and branch naming format
• Commit message format (Conventional Commits)
• Pull request rules: 300-line limit, required approvals, resolved conversations, green CI
• Work-in-progress limit (1 issue per person)
• The 30-minute blocked rule
• Definition of Done checklist
• Naming conventions for provider agnosticism (env vars, function names)
• Labels reference (area, type, status, contract)
• Pair programming points (conditional edge, SSE connection)
• Traces as the primary debugging tool

This is a documentation-only change, no code affected.

How to test

1. Read through CONTRIBUTING.md and confirm it accurately reflects how the team has been working since A-01.
2. Each of the 4 team members should review and approve this PR — this is required per the issue's acceptance criteria, not just one reviewer.

Dashboard screenshot

N/A — documentation only.

Checklist

• Traces emitted correctly (start/end) if agents were touched — N/A
• No provider SDK imported outside the gateway client — N/A
• docker compose up still works — not affected
• README updated if setup/config changed — N/A, this is a new standalone doc

[Andres-Torrez]

good job