feat: incorporate the Drupal.org AI contribution policy into bundled skills

[mglaman]

What changed

The bundled agent skills now carry Drupal.org's policy on the use of AI when contributing as operational checkpoints, so contributors using agents with this CLI adhere to it by default.

Why

Agents driving drupalorg produce code and text the user submits under their own account. The policy holds the human responsible for every line, forbids drive-by contributions, and requires disclosure of significant AI-generated work. Violations lead to account bans. None of that was represented in the skills.

How

• New shared checklist at skill-data/drupalorg-cli/references/ai-contribution-policy.md, served by skill:get drupalorg-cli --full. It covers: read the thread first, minimal and explainable diffs, no unverified dependencies, green pipeline before hand-off, never push to someone else's MR unannounced, the AI-Generated: Yes (...) disclosure format, summaries in the user's own words, and staying responsive to feedback.
• The drupalorg-cli skill and the installed discovery stub each gain a short "Contribution policy" section with the non-negotiables.
• drupalorg-work-on-issue gains [POLICY] checkpoints: fetch comments and report prior attempts in Step 1, stop when the MR author is not the user in Step 3, forbid out-of-scope refactors in the work loop, require a drafted disclosure before flipping to Needs review, and remind the user about follow-up.
• drupalorg-issue-summary-update refuses updates with no verified insight, hands over a draft to rewrite rather than paste, and adds a disclosure step.
• Workflow references and the README carry short pointers.

Known gap

--with-comments is silently ignored for GitLab work items, so the skills tell agents to fall back to glab issue view --comments for those. Tracked in #374.

Testing notes

• vendor/bin/phpunit: 212 tests pass.
• php drupalorg skill:get drupalorg-cli --full includes the new ai-contribution-policy reference.
• The CI assertion that the stub still points at drupalorg skill:get drupalorg-cli holds.

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