feat(ci): implement ADR-0022 issue traceability on pull requests #595

Summary

decisions/0022-issue-traceability-on-pull-requests.md was accepted on
2026-09-02 in #594. This tracks its six migration steps. The record is the
decision; this is the work.

The ordering matters and is part of the decision: the pull-request check lands
and is observed before the branch pattern relaxes. Doing it the other way
leaves a window with neither control active, which is the state ADR-0022 exists
to end.

Steps

1. Agree the exemption label and record it in runbooks/labels.md. It
   covers work with no owning issue: gitlink and submodule reconciliation, and
   routine dependency updates. ADR-0022 makes applying it a maintainer
   decision, so an agent may propose one but never applies it on its own
   authority.
2. Implement the pull-request traceability check in
   .github/workflows/commit-lint.yml. It passes when the pull request closes
   an issue, references one, or carries the exemption label, and it reports
   which of the three applied so an exemption is visible in review rather than
   silent.
3. Verify it on a real pull request. runbooks/branch-protection.md notes
   that GitHub only accepts status-check contexts it has already observed, so
   this has to precede step 4.
4. Register the new context in the main ruleset (19646747) alongside
   Validate Commits, Validate PR Title, and Validate Branch Name.
5. Relax Validate Branch Name to the shape check ADR-0022 defines, here
   and in z-shell/zi, which carries the same job with its patterns inlined.
   Both must move together or the two copies drift again.
6. Add the ADR-0019 pointer recording that its branch-naming clause is
   amended by ADR-0022, and update AGENTS.md where it describes issue-linked
   branch naming.

Notes

Step 2 lands in the same file as #592, which proposes test coverage for the
commit-lint patterns. Doing #592 first, or as part of step 2, is probably
cheaper than doing it afterwards: three defects have shipped in that file
(#575, the empty-pattern trap found during #586, and #587), and this adds a
fourth policy predicate to it.

#587 is an open correctness defect in the same workflow and is independent of
this work.

Follows #593 and #594.