Add a `CONTRIBUTING.md` documenting the "no lingering merge-conflict FIXMEs" policy

[fejilaup-cloud]

Labels: documentation, infrastructure
Priority: Medium
Estimated Time: 1 hour

Description:
Given how many test modules are currently disabled due to unresolved merge conflicts, a written contributor policy would help prevent recurrence: PRs should not merge with commented-out test modules or unresolved compile errors, even under time pressure.

Tasks:

• Create CONTRIBUTING.md (or add a section to an existing one)
• State the policy explicitly, referencing Add SECURITY.md with vulnerability disclosure policy #82's tracking checklist as the historical example
• Add the CI lint from Add SECURITY.md with vulnerability disclosure policy #82 to enforce it going forward

[Peculiarr7]

Hi! I’m a full-stack developer with a strong blockchain background, and I’m confident I can resolve this issue quickly and effectively. I’ll deliver a clean, reliable solution with immediate attention to detail, ensuring it’s completed as soon as possible. Please assign it to me—I’m ready to get started right away.