Add a DCO signoff check to CI and the signoff rule to AGENTS.md

[cliffhall]

Follow-up from #4867 (the pr-flow skill), which merged with its first acceptance criterion unmet: nothing fails a PR with an unsigned commit today. No DCO check ran on #4904, #4905 or #4906.

The original plan was to install the probot DCO app, but the org appears to have turned it off. The Inspector hit the same thing (modelcontextprotocol/inspector#2566), where the app was suspended and its check quietly disappeared. As there, we replace the third-party app with a check we own.

Scope

1. A DCO job in CI. For every commit in the PR's range, require a Signed-off-by: Name <email> trailer whose name and email match the commit's author or committer. Keep the app's two exemptions: merge commits and bot-authored commits. One unsigned commit fails the job, and its output names the commit and the git rebase --signoff repair.
   • typescript.yml and python.yml run on both push and pull_request, and a push job doesn't know the PR's base. A small separate workflow on pull_request (using github.event.pull_request.base.sha..head.sha, or gh api …/pulls/N/commits --paginate) is cleaner than inferring a merge-base.
   • Use contents: read only, so the job holds no credential and stays outside the SHA-pin rule (verify:action-pins).
   • Declare timeout-minutes, as scripts/lib/workflow-gate.mjs requires of every job.
   • If the logic is more than a few lines, put it in scripts/verify-dco.mjs with a sibling verify-dco.test.mjs, like the other guards.
   • Per AGENTS.md, a check added to CI joins local:gate:stages in the same change. Locally it can check origin/v2/main..HEAD, which also catches an unsigned commit before it is pushed.
2. Verify it: push an unsigned commit to a throwaway PR against v2/main and confirm the job fails. Then repair it with git rebase --signoff and confirm it passes.
3. Make it required. Add the job's check as a required status check in a ruleset that covers v2/main. No ruleset applies to v2/main today, and main's rulesets require no status checks, so this is a repo-admin settings change, not something the PR can do. Consider main too.
4. Update the docs, in the same PR as the job:
   • add the signoff rule to AGENTS.md, which only states rules that are true of the repo today;
   • rewrite .claude/skills/pr-flow/SKILL.md step 3 so it describes the new job instead of the probot app: delete the "⚠️ The probot DCO app is not installed on this repo yet" paragraph, and drop the .github/dco.yml / remediation-commit / override-button discussion, which only applies to the app;
   • realign the AGENTS.md skills-index table: the issue-triage and security-advisory rows were added without padding to avoid stacking conflicts.

Acceptance criteria

• A CI job fails a PR that has an unsigned commit, names the commit and the repair, and passes once it is signed off.
• The check runs in local:gate:stages, and its script (if any) has unit tests.
• The check is a required status check on v2/main.
• AGENTS.md states the signoff rule, and pr-flow describes the job, not the app.
• The skills-index table is Prettier-clean.

Part of #4858.