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.

tests/pr-workflow-approval.cjs

@@ -333,6 +333,7 @@ test('only same-repository sync branches queue behind an in-flight build', () =>
   assert.equal(cancels('omacom/omarchy-pkgs', 'auto/sync-upstream'), false);
   assert.equal(cancels('omacom/omarchy-pkgs', 'auto/sync-upstream-ttfx'), false);
   assert.equal(cancels('omacom/omarchy-pkgs', 'auto/sync-rebuilds'), false);
+  assert.equal(cancels('omacom/omarchy-pkgs', 'auto/track-branches-omarchy-dev-omarchy-settings-dev'), false);
   assert.equal(cancels('omacom/omarchy-pkgs', 'ttfx/fix'), true);
   assert.equal(cancels('someone/omarchy-pkgs', 'auto/sync-upstream'), true);
   assert.equal(cancels(undefined, ''), true); // workflow_dispatch