Require downstream release review and published-artifact conformance

[rmcdaniel]

Changes

• Make downstream impact, necessary releases/deployments, and explicit no-change decisions part of every release handoff.
• Require affected conformance experiments against the actual published tuple, not source CI or an older result.
• Make notifications and approved intake reviews proactive, with read/done hygiene and retained ownership of unfinished work.

This is contributor guidance in the existing AGENTS.md, not a new release controller or internal operating system. Cloud-specific decisions remain private. No runtime behavior changes.

Validation: reviewed against the existing release/conformance/intake rules; git diff --check passes.