feat(setup-steward): auto-overwrite committed bootstrap skill on upgrade

[potiuk]

Summary

When /setup-steward upgrade refreshes the snapshot to a new framework version, automatically replace the adopter-side committed setup-steward skill (the only framework artefact they commit) with the version from the just-installed snapshot.

Why

The committed setup-steward is the only framework skill an adopter copies into their repo (Golden rule 6: "copy this skill, symlink the rest"). When the framework's setup-steward evolves — new sub-action, lock-format change, drift-detection refinement, recipe addition — the adopter's copy must follow, or the bootstrap on a fresh clone will run old logic against a new snapshot.

Before this PR, upgrade only surfaced the drift between the committed copy and the snapshot. After this PR, upgrade actually fixes it.

Behaviour change

upgrade.md adds a new Step 6b — Overwrite the committed setup-steward skill from the new snapshot (between symlink refresh and local-lock update):

1. Diff the adopter-side committed setup-steward against the snapshot's setup-steward.
2. If the adopter has local modifications beyond the snapshot's content — surface the diff and stop. Do NOT silently overwrite local work. The user either:
   • confirms the modifications can be discarded, OR
   • upstreams them as a framework PR first, OR
   • defers the bootstrap-skill update to a later upgrade.
3. Otherwise, cp -r the snapshot's setup-steward over the committed copy. The new content lands in git status for the user to commit as part of the upgrade PR.

Layout-aware:

• Flat: cp into .claude/skills/setup-steward/.
• Double-symlinked: cp into .github/skills/setup-steward/. The .claude/skills/setup-steward symlink keeps pointing at the right (now-updated) location.

verify.md Check 7 ("setup-steward up to date") updated to distinguish the two drift directions:

• Snapshot newer than committed (typical post-framework-upgrade case before the adopter has rerun /setup-steward upgrade) → point at upgrade's Step 6b auto-overwrite.
• Committed newer than snapshot (anti-pattern: adopter modified bootstrap directly) → upstream the modifications as a PR against apache/airflow-steward, or revert to use .apache-steward-overrides/ instead.

upgrade.md Step 2 ("Surface what changed") updated to reflect that bootstrap-skill drift is no longer a manual decision — Step 6b handles it.

Test plan

• All prek run --all-files hooks pass (markdownlint, typos, doctoc, check-placeholders)
• Step 6b heading anchor (#step-6b--overwrite-the-committed-setup-steward-skill-from-the-new-snapshot) is reachable from verify.md's back-reference
• Reviewer eyeballs the local-modification protection flow: cardinal sin would be silently overwriting work the adopter painted into the bootstrap copy
• CI link-check (lychee) passes

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