Unassign inactive reviewers on pull requests

[Lacah]

Problem
Reviewers sometimes assign themselves to a PR and then go silent, leaving the review stalled.

Proposal
Add a GitHub Action that:

• Runs on a schedule (e.g., every 15 min)
• Unassigns reviewers after X minutes of inactivity
• Only removes them if:
  • No comments or file reviews were made
  • The PR is still open
• Optionally comments that they were unassigned

Goal
Keep pull requests flowing by automatically freeing up inactive reviewers.