Automate .authors.yml updates before release

[danyeaw]

What is the idea?

Add a conda-owned action that scans recent commits, updates .authors.yml for new contributors and alternate emails, and opens or updates a PR. A check-only mode validates without writing. Rever still handles .mailmap and AUTHORS.md at release time.

Why is this needed?

Stale .authors.yml is a common cause of rever failures during release week. Fixing it mid-release involves painful back-and-forth between .authors.yml and .mailmap. Keeping authors metadata current on main avoids that firefighting.

What should happen?

• Merge Add action for preparing authors actions#470 (prepare-authors)
• Add a weekly prepare-authors workflow template to infrastructure sync
• Update RELEASE.md and RELEASE_ON_DEMAND.md for continuous authors maintenance and what rever still owns at release
• Pilot in conda/conda and conda-build, then roll out to other rever-based repos

Additional Context

Complements #1378. Default: weekly prepare on main, scanning since latest tag. Doc updates should land after or alongside infrastructure#1356 to avoid RELEASE.md conflicts.