docs(contributing): replace GPT-5.4-only policy with disclosure-only

[flyinghanger]

Summary

Closes #724.

Replace the GPT-5.4-only AI-Generated Code policy with a disclosure-only line. Rationale is in #724 (one-line recap: the pinned model is no longer current SOTA, detection isn't enforceable, and the policy penalizes honest disclosure rather than filtering for quality).

This adopts option 2 from the issue (disclosure-only). Happy to switch to option 1 (drop the section entirely) or option 3 (rolling allowlist) if you prefer a different shape.

Diff

 ## AI-Generated Code Policy (Vibe Coding)

-If you are using AI tools to generate code ("Vibe Coding"), please note:
-
-**We only accept code generated by the GPT-5.4 model.**
-
-Pull requests containing code generated by other AI models (including other GPT versions, Claude, Gemini, or any other LLM) will be rejected. This policy helps us maintain consistency and quality standards across the codebase.
-
-If you're unsure whether your AI-assisted code meets this requirement, please open an issue to discuss before submitting.
+If you used AI tools to generate code ("Vibe Coding"), please disclose the model in your PR description. Code is judged on merit — focused scope, tests, and the other expectations below apply equally regardless of how the code was authored.

Test plan

• Docs-only change; no code paths affected.
• CONTRIBUTING.md renders correctly on GitHub.

Disclosure

Per the new policy (and the spirit of the old one): drafted with Claude (Opus 4.7), reviewed and submitted by me.

[github-actions]

Findings

• No issues found.

Summary

• Review mode: initial
• Documentation-only change in CONTRIBUTING.md; no correctness, security, data loss, performance, or maintainability issue found in the modified line.
• Residual risk: wording/policy preference only; no runtime coverage needed.

Testing

• Not run (automation)

HAPI Bot