feat: adopt apache/airflow-steward via snapshot+overrides mechanism

[potiuk]

Summary

Apache Airflow becomes an adopter of the apache/airflow-steward framework using the snapshot + agentic-overrides adoption mechanism. The previous in-repo PR-skill copies (pr-triage, pr-stats, maintainer-review) are replaced by the framework's renamed pr-management-* family delivered through the framework.

This supersedes the closed #66283, which used a git-submodule design that we ultimately rejected.

How adoption works

The framework lives in a gitignored .apache-steward/ directory in this repo, refreshed by the framework's own setup-steward skill. The only framework artefact this PR commits is that bootstrap skill itself — at .github/skills/setup-steward/ (matching airflow's existing .claude/skills/<n> → .github/skills/<n>/ double-symlink convention).

On a fresh clone, the framework's other skills are not yet on disk. To populate them:

# tell your agent: "adopt apache-steward in this repo"
# OR invoke /setup-steward directly

The setup-steward skill then:

• Downloads the framework into .apache-steward/ (gitignored, refreshed via /setup-steward upgrade).
• Creates gitignored symlinks under .github/skills/<framework-skill>/ → .apache-steward/.claude/skills/<framework-skill>/.
• Wires up matching gitignored symlinks under .claude/skills/ so Claude Code picks them up.
• Recreates everything if a worktree is checked out off this repo (post-checkout hook).

What lands committed

File	Purpose
.github/skills/setup-steward/ (+ .claude/skills/setup-steward symlink)	The bootstrap skill — the only framework artefact this repo commits.
.gitignore	Gitignore .apache-steward/, the runtime framework symlinks; remove old in-repo skill exemptions; add setup-steward exemption.
.apache-steward.lock	Pins the snapshot to commit 84852ca1a0…. Updated by /setup-steward upgrade.
.apache-steward-overrides/README.md	Override-directory scaffold. Adopter-specific deviations live here.
.markdownlintignore	Skip the framework's skill files (they follow the framework's markdownlint conventions; already linted upstream).
CLAUDE.md (= AGENTS.md)	Short adoption note explaining the bootstrap flow.
contributing-docs/25_maintainer_pr_triage.md	Top-of-doc notice that skills now live in the framework + path updates pr-triage → pr-management-triage etc.

What got deleted

• .github/skills/pr-triage/, .github/skills/pr-stats/, .github/skills/maintainer-review/ — replaced by pr-management-{triage,stats,code-review} from the framework.
• The corresponding .claude/skills/ symlinks.

What stays untouched

• .github/skills/aip-user-stories/, .github/skills/airflow-translations/, .github/skills/prepare-providers-documentation/ — airflow-only skills that don't migrate to the framework.
• The .claude/skills/aip-user-stories symlink.

Test plan

• prek run passes locally (markdownlint, codespell, license-header, …)
• setup-steward skill registers in Claude Code's available-skills (live discovery picks it up)
• .gitignore correctly ignores .apache-steward/ and the gitignored symlinks
• Reviewer runs /setup-steward against a fresh clone of this branch to verify the end-to-end adoption flow
• Reviewer eyeballs the deletion of pr-triage/pr-stats/maintainer-review for any in-repo references not covered by the contributing-docs path update
• CI passes

Out of scope

• Deeper rewrite of contributing-docs/25_maintainer_pr_triage.md to reflect the framework's pr-management-* skill structure (steps, decision tables, etc.). This PR does the path renames + top-of-doc notice; a follow-up can do the substantive update once maintainers have run /setup-steward and read the new framework's skill files.
• Re-adopting the security-issue handling family (those skills run from the private apache/airflow-s security tracker, not from this public upstream).

🤖 Generated with Claude Code

[Srabasti]

Hello @potiuk
Looks like the new file ".markdownlintignore" is added, however ".rat-excludes" is not updated to include it and tell RAT to ignore it.
Shall we update it instead of adding an Apache License header to it?

[potiuk]

No no... I will have to redo it