feat: label merge-queue batch PRs with a pill and fix detection on the current DOM

A merge-queue batch PR now renders a "Merge-queue batch PR" pill in the injected
row, so the button-less batch shape reads as intentional rather than empty. The
pill uses the buildStatePill visual language — informational accent-blue, no pulse.

Batch detection is repointed at the current React PR DOM, which no longer serves
the selectors it relied on: isPullRequestAuthoredByMergify scopes to the opening
comment (.js-command-palette-pull-body) instead of .gh-header-meta, and getBaseRef
reads the base branch off the first branch-name link's /tree/<branch> href instead
of .commit-ref.base-ref. Both keep the legacy selector as a fallback for the older
DOM era. Adds a real batch-PR fixture (github_pr_batch_open) recorded on today's
DOM, with fixture-backed tests for detection, base ref, the pill, the button-less
row, and the absent dequeue button.

MRGFY-7874

Test plan:
- npx jest (283 passing) — detection/base-ref/pill/row on the real fixture; legacy
  synthetic + old-fixture paths still green.
- npx biome check — clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KJbLFjMX9mb8o5T3JXTAXL

Change-Id: I3d56072626baaba4158792ac762c3878e6582181

Author: JulianMaurin