Standardize Alembic migration descriptions and add a style lint

[FrankYang0529]

Why

• Migration descriptions are user-facing: they are rendered into the migrations-ref.rst documentation table and printed by alembic history / airflow db migrate.
• Out of 124 core migrations, 15 descriptions start with a lowercase letter and 2 use non-imperative mood (Added ..., Adding ...), making those listings inconsistent.

How

• Add rule MIG004 to the check-migration-patterns prek hook: the description must start with a capital letter, use the imperative mood, and end with a period.
• Extend the hook to providers/*/migrations/versions/. fab and edge3 have their own migration trees and docs tables.
• Normalize the 17 non-conforming descriptions and regenerate migrations-ref.rst via the update-migration-references hook.
• Fix the contributing-docs example

• closes: Inconsistent capitalization and verb tense in Alembic migration descriptions #69496

---

Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[ashb]

When did this convention if creating our own ruff-style rule codes that aren't part of ruff come around? I'm not a huge fan of it, and I'm aware of any discussion about it.

[FrankYang0529]

When did this convention if creating our own ruff-style rule codes that aren't part of ruff come around? I'm not a huge fan of it, and I'm aware of any discussion about it.

Rules MIG001, MIG002, and MIG003 were introduced from #64972. I cannot find discussion in dev mail list, either. IIUC, the ruff hasn't supported custom linter yet. We may still need to rely on this to do the check.

https://docs.astral.sh/ruff/faq/#can-i-write-my-own-linter-plugins-for-ruff

[pierrejeambrun]

LGTM, thanks.

Fair question from Ash, but outside the scope of this PR I suppose. (good potential follow up though)

[github-actions]

Backport successfully created: v3-3-test

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
✅	v3-3-test