Add accessibility-frontend-reviewers workflow

[MReschenberg]

Summary

• Adds bot/code_review_bot/tasks/a11y_frontend.py, a self-contained module implementing the #accessibility-frontend-reviewers review group workflow
• Wires it into PhabricatorReporter.publish() with a single import and call

Workflow

1. #accessibility-frontend-reviewers group is tagged for the first time on a revision
2. A static comment is posted to the revision containing some instructions.
3. The reviewer group is removed from the revision
4. If the author adds the review group back after removal, no action is taken by the automation (no comment posted, the review group stays assigned to the revision).

Comment posted containing the following:

• You've tagged the accessibility team for review.
• If you've already spoken to a team member about your request, please request review from the specific team member you spoke to.
• Otherwise, please do ALL of the following:
  • Post a comment describing the specific accessibility behaviour you're looking for feedback about
  • Attach screenshots showing behaviour before and after your patch (if this is a visual change)
  • Post a description of screen reader output/behaviour before and after your patch (if this change affects assistive technology users)

If the group is added by a member of the accessibility team, no comment is posted and the group is left in place; team membership is resolved dynamically from Phabricator at runtime so no names or PHIDs are hardcoded. On re-add, the automation verifies the author has followed the instructions by checking whether they posted a comment after the bot's removal or tagged an individual team member as a reviewer; if neither condition is met, the comment is re-posted and the group is removed again.

Test coverage

bot/tests/test_a11y_frontend.py:

• First-time tagging by a non-team-member: comment is posted and group is removed
• Tagging by an accessibility team member: no action taken
• Re-add after the author posts a followup comment: group stays
• Re-add after the author tags an individual team member: group stays
• Premature re-add (no followup comment, no individual team member tagged): comment is re-posted and group is removed again
• Group not present on the revision: no API calls beyond the initial lookup
• Group PHID not found in Phabricator: warning logged, nothing else called

[MReschenberg]

brb fixing up those checks :)