Agentic software factory Part 14: v2/main → main milestone release flow and release skill

[cliffhall]

Part of the agentic software factory tracker #4858, Wave 5. Proposed as S12 in docs/agent-guidance-inception.md (PR #4861) §9, from #4859.

Depends on

After #4472 (changesets + Release-triggered publishing), Part 11 (local:gate) and Part 12 (knowledge skills: the ledger uses client-smoke).

Scope

• Preparation PRs, all on v2/main, all merged before the merge PR opens:
  • the audit report (npm and uv/pip), with any forced fixes as their own commits
  • the version bumps, which Agentic software factory Part 13: v2 release pipeline, Phase 2: semver via changesets (TS), GitHub-Release-triggered publishing #4472 makes two separate PRs: the changesets "Version Packages" PR for TS and the prepare-release CalVer PR for Python. A milestone touching both ecosystems therefore has up to three preparation PRs.
• The merge PR: a pure v2/main → main merge, whose tree hash matches origin/v2/main, with a release ledger artifact linked from it. The ledger records local:gate, pack:verify for each package, each server-facing milestone issue exercised via client-smoke, and a targeted probe for each issue with no client surface (docs, skills, workflows).
• A maintainer then publishes the GitHub Release.
• Split release.yml so build, install and verify run in jobs without id-token: write. Only the publish job holds it (inspector#2483).
• Pin actions in every credentialed job before the first release through this flow, enforced by a ported verify:action-pins. That covers release.yml's publish jobs, every job whose artifact a credentialed job downloads (the build/pack jobs the split introduces), and claude.yml (id-token: write, ANTHROPIC_API_KEY). Add the AGENTS.md SHA-pinning rule.
• The release skill is name-only (disable-model-invocation: true).
• Rewrite RELEASING.md to describe the merged state.

Acceptance criteria

• verify:action-pins passes, and fails on a tag-pinned action in any credentialed or artifact-producing job.
• One milestone has been released end to end through the skill.
• The ledger is linked from the merge PR.
• RELEASING.md describes the merged flow.

Branch

Targets v2/main. Closes #N won't auto-close an issue on a non-default branch, so close this one by hand on merge and move the card to Done.