feat(skills): add release-captain skill for the weekly release cadence

[matthewgrossman]

Summary

Add an internal release-captain skill with a checklist for the weekly RFC 0014 release train: Monday pre-flight, the Tuesday stable cut, and recovery. Every action that changes something (tags, workflow runs, merges, Slack/GitHub posts) requires explicit maintainer approval.

Related Issue

Closes #4433

Changes

• Add .agents/skills/release-captain/SKILL.md:
  • Monday: find the train and its latest pre-release; check the qualification summary for the latest attempt against the tag and commit; triage the advisory compatibility verdict and report; check the dev canary; draft the CNCF coordination post with ready PRs and a merge cutoff before the 14:00 Europe/Zurich Auto-Tag run.
  • Tuesday: select the final qualified pre-release; check the version, commit, and existing stable tags before tagging its exact commit; watch Release Tag; check publication; seed the next train's -pre.1 on a later commit with green CI, then dispatch it.
  • Recovery: stuck or failed pre-release and stable runs. Don't re-dispatch while a run for the same tag might still start (Release Tag has no concurrency guard), and never repoint a stable tag.
• Register the skill in the CONTRIBUTING.md skills table and the sync-agent-infra maintenance map.

This complements #4335 (release-openshell) and changes no workflows. If #4335 lands, the skill's note that pushing a -pre.* tag doesn't start Release Tag needs a one-line update. If #4419 renames qualification jobs or artifacts, the commands will need the same update.

Testing

• Checks appropriate to the affected code and behavior pass
  • markdownlint-cli2 passes on CONTRIBUTING.md. The repo's markdownlint config excludes .agents/**, so the skill file wasn't linted.
  • Ran the skill's read-only commands against real runs: the pre-release run lookup by commit (v0.1.4-pre.3 → run 38248106021), downloading the latest attempt's qualification and compatibility artifacts, jq field extraction (current_profile_passed, review.verdict), the pre-tag checks (tested on v0.1.4-pre.3 as the candidate; the stable-tag check fires on the v0.1.3 commit), and the job-failure filter on the v0.1.3 release run 37947117841.
  • Checked that the relative RFC links resolve.
• Unit tests added/updated (if applicable)
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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