Add contributor result submission skill

[isPANN]

Summary

• add a repository-local $submit-benchmark-result skill for non-maintainer submitters
• delegate upload/authentication from both benchmark-running skills to the new skill
• document the current legacy intake credentials and the GitHub-backed Cloudflare Access direction
• add structural tests for validation-before-upload and GitHub credential isolation

Authentication boundary

The deployed client still supports PRB_API_KEY only. The skill therefore does not pretend
that a GitHub login is enough today. It prefers an application-scoped Cloudflare Access token
once both the client and Worker support it, rejects GitHub PAT/gh auth token substitution,
and otherwise stops safely when no locally configured legacy credential exists.

Verification

• quick_validate.py .agents/skills/submit-benchmark-result
• forward-tested with an existing valid result and no intake credential
• forward-tested with no result yet (routes to $run-benchmark before any paid run)
• uv run pytest -q (223 passed, 4 skipped)