MTE-6112 Pin the canary rebase to 07:00 America/New_York year-round

GitHub cron is UTC-only, so a single '0 11 * * *' entry meant 07:00 local during
EDT but 06:00 during EST. Now fires at both 11:00 and 12:00 UTC with a guard step
that stops the run unless the America/New_York hour is 07, so the job lands at
07:00 local regardless of DST. Manual dispatch bypasses the guard.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: clarmso

.github/workflows/xcode27-canary-rebase.yml

@@ -13,9 +13,11 @@ name: Rebase Xcode 27 canary branch onto main
 
 on:
   schedule:
-    # 11:00 UTC = 07:00 America/New_York while EDT is in effect (06:00 during EST;
-    # GitHub cron has no timezone support, so the local hour shifts with DST).
-    - cron: '0 11 * * *'
+    # GitHub cron is UTC-only, so fire at both candidate hours and let the job's
+    # timezone guard drop the wrong one. That pins the run to 07:00
+    # America/New_York year-round instead of drifting an hour with DST.
+    - cron: '0 11 * * *' # 07:00 America/New_York during EDT
+    - cron: '0 12 * * *' # 07:00 America/New_York during EST
   workflow_dispatch:
     inputs:
       branchName:
@@ -32,6 +34,18 @@ jobs:
     env:
       CANARY_BRANCH: ${{ github.event.inputs.branchName || 'cs/mte-6112-xcode27-firefox-only' }}
     steps:
+      # Both cron entries fire year-round; only the one matching 07:00 local
+      # should proceed. Manual dispatch always proceeds.
+      - name: Skip unless it is 07:00 America/New_York
+        if: github.event_name == 'schedule'
+        run: |
+          LOCAL_HOUR=$(TZ=America/New_York date +%H)
+          echo "07:00 target; America/New_York hour is ${LOCAL_HOUR}"
+          if [ "$LOCAL_HOUR" != "07" ]; then
+              echo "Not 07:00 local (DST duplicate); stopping."
+              exit 1
+          fi
+
       - uses: actions/checkout@v4
         with:
           # Full history: a rebase needs the merge base with main.