Adopt apache/airflow-steward framework via submodule

[potiuk]

Summary

Apache Airflow becomes an adopter of the apache/airflow-steward framework — a project-agnostic skill library for ASF-project automation. The framework lands as a git submodule at .apache-steward/apache-steward/; the per-project adopter configuration sits alongside it under .apache-steward/.

This is the first of three airflow-side PRs that complete the migration:

1. (this PR) Add the submodule + adopter config. Existing .claude/skills/ symlinks and .github/skills/{pr-triage,pr-stats,maintainer-review} stay in place — nothing breaks for current users.
2. Follow-up — replace the .claude/skills/ symlinks with submodule-pointing ones; delete the duplicate .github/skills/ copies.
3. Follow-up — update contributor docs to reference the framework.

Why

The pr-triage, pr-stats, and maintainer-review (renamed pr-maintainer-review) skills were lifted into the framework over the last week (apache/airflow-steward#33) so other ASF projects can adopt the same playbook. Airflow now consumes them via submodule rather than maintaining its own copies under .github/skills/. Project-specific knobs (committers team, area-label prefix, comment-template URLs, CI-check map, review-criteria source files) live in .apache-steward/ per the framework's adopter contract.

The security workflow is not adopted here — those skills run out of the private security-tracker repo apache/airflow-s, not out of the public upstream — so no security-workflow project-config files (canned-responses.md, scope-labels.md, release-trains.md, etc.) live in this directory.

What's in .apache-steward/

File	Purpose
project.md	Identity (Apache Airflow / apache/airflow), upstream repo, sibling-file pointers
pr-triage-config.md	Committers team (apache/airflow-committers), area-label prefix (area:), project labels (ready for maintainer review, etc.), grace windows
pr-triage-comment-templates.md	Comment-body URLs (PR quality criteria, two-stage triage rationale), AI-attribution footer, project display name
pr-triage-ci-check-map.md	CI-check pattern → category + doc-URL map (lifted from what pr-triage/comment-templates.md currently embeds inline)
pr-maintainer-review-criteria.md	Review-criteria source files (code-review.instructions.md, AGENTS.md, per-area AGENTS.mds), security-model calibration, backport pattern, section anchors

Operational notes

• Fresh clones / parent git pull: run git submodule update --init --recursive. Wire it as a post-merge hook to make it automatic — the framework's setup-verify-steward skill checks integration and reminds.
• Submodule pointer: pinned at the latest framework main (commit 9774cc9, includes the four steward PRs that lifted + documented these skills).
• No skill behaviour change yet: the existing .github/skills/{pr-triage,pr-stats,maintainer-review} and the symlinks under .claude/skills/ are untouched. Skills run from the same content they always did. The cutover is in the follow-up PR.

Test plan

• All prek run hooks pass (markdownlint, codespell, license-header, etc.)
• .apache-steward/ content describes airflow accurately (committers team, area-label prefix, URLs, AGENTS.md paths)
• Reviewer eyeballs .apache-steward/project.md for accuracy
• Reviewer confirms the existing .claude/skills/ and .github/skills/ setup still works (no behaviour change in this PR)
• CI passes (lychee link-check, full prek)

🤖 Generated with Claude Code

[potiuk]

Closing... I have a way better idea how it can be done :) - https://lists.apache.org/thread/sjnd6s0wczkymhcpslrq1oj51ndr3sbo

[potiuk]

Note for future archaeologists: closed because the adoption design pivoted. The framework will not be consumed via git submodule; instead an adopter copies a single setup-steward skill into their repo, and that skill manages a .gitignored snapshot of the framework + symlinks the framework's other skills into the adopter's skill directory (which symlinks are also .gitignored). Local workflow modifications live as agent-readable overrides under .apache-steward-overrides/ in the adopter repo. See the upcoming setup-steward skill PR on apache/airflow-steward for the full design + onboarding flow.