Add a policy on AI-generated / AI-assisted contributions

Part of #6393.

Problem

Nothing in CONTRIBUTING.md or the pull request template addresses AI-generated or AI-assisted contributions. As we label more issues Help-Wanted, we want to welcome responsible AI-assisted work while guarding against low-effort, unreviewed AI-generated PRs.

Proposed change

Add a section to CONTRIBUTING.md covering:

• Contributors are fully accountable for AI-assisted code as if they wrote it themselves - they must understand it, and it must meet the same conventions, testing, and evidence bar as any other contribution.
• Material use of AI generation should be disclosed in the PR.
• Low-effort, unreviewed, or unverifiable AI-generated PRs may be closed.

Align terminology with the existing spec attribution guidance in .github/instructions/specs.instructions.md, which already documents crediting AI assistance in the spec author field.

Notes for engineers / review

The wording should be reviewed by the team (and, if appropriate, CELA) before merging. This issue creates the draft and the placement; final language needs sign-off.

Acceptance criteria

• CONTRIBUTING.md includes an AI-assisted contribution policy consistent with the PR template disclosure (see the PR-template sub-issue).

(Created with GitHub Copilot assistance.)

Read CONTRIBUTING.md and the existing attribution guidance in .github/instructions/specs.instructions.md first. Add the requested AI-assisted contribution policy to CONTRIBUTING.md, aligning its terminology with the existing guidance and PR-template disclosure. Done means the acceptance criterion is met and the wording has received the required team review, with CELA consultation if appropriate.