docs(triage): define agent-first label taxonomy

[shtse8]

Description

Adds the initial org-level SSOT for the Agent-first Signal/Triage label taxonomy and triage comment convention.

This implements the process-fix path from #4 without changing repository settings or applying labels automatically.

Related Issues

• Closes Agent-first triage: standardize cross-repo label taxonomy and stale/blocked queues #4

Type of Change

• 📚 Documentation update
• 🔧 Build/tooling change/process metadata guidance

Testing

Test Coverage

• Manual review completed

Test Details

git diff --cached --check

Checklist

Code Quality

• Self-review completed
• No unnecessary debug code

Documentation

• README update not needed
• New docs page added

Dependencies

• No new dependencies added

Breaking Changes

• No breaking changes

Security Considerations

• No secrets or credential material included
• No repository settings changed
• No merge/deploy action taken

Additional Notes

Signal/Triage guardrails preserved:

• no direct push to main;
• branch + commit + push + PR only;
• repo/project boundaries remain separate;
• labels are documented as metadata only, not authority to merge or deploy.