Update apache-steward snapshot to 5c211a4 (#67624)

* Update apache-steward snapshot to 5c211a4

Bumps the local apache-steward snapshot from 339d3eb to 5c211a4 (22
upstream commits). The only committed change in this PR is a
1-line frontmatter addition (capability: capability:setup) to
.github/skills/setup-steward/SKILL.md, propagated from the new
framework version via /setup-steward upgrade. Everything else
lives in the gitignored .apache-steward/ snapshot.

Highlights from upstream (apache/airflow-steward):

- pr-management-triage: session-history gist persistence Step 6b
  (apache/airflow-steward#343), four classifier heuristic fixes
  (apache/airflow-steward#344), fetch-all-upfront pattern
  (apache/airflow-steward#346)
- security-issue-triage: fetch-all-upfront analogue
  (apache/airflow-steward#347)
- Framework labels + capability taxonomy (apache/airflow-steward#340) —
  the source of the frontmatter line in this PR
- New skill pairing-self-review and tool spec-status-index
- claude-code pin 2.1.141 -> 2.1.150

/setup-steward upgrade ran cleanly locally: snapshot refreshed,
symlinks resolve, post-checkout hook in sync,
sandbox-add-project-root reconciled across 3 worktrees.
.apache-steward.local.lock updated to fetched_commit 5c211a4.
All .apache-steward-overrides/ files unchanged.

* Gitignore .apache-steward.session-state.json

Adds the per-machine session-state file to .gitignore. The file is
written by steward skills that maintain adopter-local persistence
anchors — currently pr-management-triage Step 6b's session-history
gist URL (apache/airflow-steward#343), but the structure is
deliberately shared so other skills can add their own keys later.

The file is per-user, per-machine state; it should never be
committed even when a contributor stages everything with `git add -A`.