Contribution gate workflow allowlist follow-up (restored from #2086)

[Hmbown]

Restored from deleted issue #2086 because @nightt5879 had explicitly offered to take the work, and @kolief later added a useful implementation audit.

Preserved contributor intent

@nightt5879 offered to open a focused PR for contribution gate workflows, an allowlist file, and CONTRIBUTING.md docs.

@kolief later reported that the relevant workflow pieces may already exist and appear complete:

• PR gate workflow;
• issue gate workflow;
• maintainer approval workflow;
• .github/APPROVED_CONTRIBUTORS;
• CONTRIBUTING.md documentation.

Maintainer note

Please verify the current repository state before coding. If the gate is already implemented, this issue should close with exact file/test evidence. If gaps remain, keep the PR narrow and focused on the missing workflow/docs pieces only.

Acceptance target

The contribution gate should be clear, respectful, and documented: no hostile language, no weekend clause, and no broad automation changes beyond the allowlist/gate flow.

[Hmbown]

@nightt5879 @kolief I’m sorry: I was doing public issue cleanup to pull back broad Hunter-authored roadmap items and went overboard. The original #2086 was deleted even though @nightt5879 had claimed it and @kolief had added useful implementation analysis.

I restored the follow-up here so that work and context are still visible. Before anyone codes against it, we should verify whether the current workflows and docs already satisfy the original issue, then either close this with evidence or keep the follow-up very narrow.