chore(workflow): align RFC, gator, stale, and contributor guidance

[purp]

Description

Align RFC handling, gator, stale handling, PR/issue guidance, and contributor documentation with the new label contract under epic #3807. This is the fourth layer of the initial draft PR stack (#3820–#3823).

Context

rfc/README.md describes needs-rfc; the gator skill discovers untriaged issues by the old intake label; the stale workflow exempts the old states and agent chain. AGENTS.md, CONTRIBUTING.md, and related contributor skills explain the old workflow. The proposal keeps gator:* meanings and does not require issue workflow labels to be mirrored on linked PRs.

Definition of Done

• RFC guidance and relevant skills use the agreed needs:rfc rule and retain maintainer control of RFC numbering.
• Gator's issue discovery and handoff agree with the new issue labels while its own gator:* state machine retains its meaning.
• Stale exemptions and labels use the new scheme without changing staleness timing or closure policy.
• AGENTS.md, CONTRIBUTING.md, README.md, relevant skills, and any affected templates tell one consistent workflow story; run the sync-agent-infra checks.
• PR handling relies on the linked issue for issue-workflow labels and does not require duplicate labels on PRs.