feat: improvements to release automation

[kim-em]

This PR improves the scripts assisting with cutting Lean releases (by reporting CI status of open PRs, and adding documentation), and adds a .claude/commands/release.md prompt file so Claude can assist.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 7632cefa8791817df6c27e0b04ee39cd45330eaf --onto 14ff08db6f651775ead432d367b6b083878bb0f9. You can force Mathlib CI using the force-mathlib-ci label. (2025-10-14 22:46:59)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 7632cefa8791817df6c27e0b04ee39cd45330eaf --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-10-14 22:47:00)