Document post-merge GitHub release follow-up

[palewire]

Summary

• Document the required post-merge release order in AGENTS.md.
• Make RELEASING.md require an approved tag-triggered package publication before creating the public GitHub Release.
• Replace the stale CONTRIBUTING.md walkthrough with a pointer to the authoritative checklist.

Validation

• git diff --check
• make docs-check
• Pre-commit hooks and ty passed during commit

Assumptions

• The existing tag-triggered PyPI workflow remains unchanged; this is procedural guidance, not new automation.
• Maintainers may use gh or the GitHub UI, but release/tag/package publication actions still require explicit human approval.