Give each tracked package its own PR, and let a fresh branch tip fall back

The branch tracker stalled omarchy-dev and omarchy-settings-dev twice in a
week:

- All ~50 auto_merge packages rode one PR, so omasnap-git's failing smoke
  suite kept the quattro pins unmerged from Oct 4 to Oct 8 (#788). Each
  package now gets its own PR on auto/track-branches-<package>; packages
  pinned from the same branch share one, since they move in lockstep. A run
  closes tracker PRs that master has moved past, and the old single PR.
- A branch tip younger than min_release_age held the whole branch, so a
  run that landed shortly after a quattro commit pinned nothing. The watch
  now falls back along first-parent history to the newest commit that has
  cleared the window, measured from clone time so siblings agree.
- The every-2-hours schedule fired every 6 to 9 hours in practice; run it
  hourly.

.github/workflows/build-pr.yml

@@ -21,7 +21,7 @@ on:
         required: true
 
 # A new push normally cancels the PR's in-flight build. The bots' branches
-# (auto/sync-* and auto/track-branches) are the exception: they are
+# (auto/sync-* and auto/track-branches-*) are the exception: they are
 # force-pushed with fresh upstream releases several times a day, which kept
 # cancelling multi-hour aarch64 builds before they could finish. There the
 # newest run waits instead (GitHub keeps at most one pending run per group,
@@ -31,7 +31,7 @@ on:
 # runs at once; cancelling the first left a red `result` on the PR (#901).
 concurrency:
   group: build-${{ github.event.pull_request.number || github.run_id }}
-  cancel-in-progress: ${{ !(github.event.pull_request.head.repo.full_name == github.repository && (startsWith(github.head_ref, 'auto/sync-') || github.head_ref == 'auto/track-branches')) }}
+  cancel-in-progress: ${{ !(github.event.pull_request.head.repo.full_name == github.repository && (startsWith(github.head_ref, 'auto/sync-') || startsWith(github.head_ref, 'auto/track-branches'))) }}
 
 jobs:
   # Builds cost real machines, so they run only for trusted authors: