Harden RC release schedule against dropped cron runs

.github/workflows/release-rc.yml

@@ -3,7 +3,7 @@ name: Schedule RC Release
 on:
   workflow_dispatch:
   schedule:
-    - cron: '0 3,15 * * *'
+    - cron: '7,17,27 3,15 * * *'
       timezone: 'America/Los_Angeles'
 
 permissions:
@@ -36,11 +36,35 @@ jobs:
           git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
 
       - name: Sync with latest main
-        run: git pull --ff-only origin main
+        run: |
+          git fetch origin main --tags
+          git checkout main
+          git reset --hard origin/main
+
+      - name: Compute RC slot
+        id: slot
+        run: |
+          slot=$(TZ=America/Los_Angeles date '+%Y-%m-%d-%H')
+          echo "value=$slot" >>"$GITHUB_OUTPUT"
+
+      - name: Skip if this PT release window already ran
+        id: existing
+        run: |
+          # Why: GitHub can delay or drop schedule events, especially near the
+          # start of the hour. We schedule multiple attempts inside each target
+          # PT hour and make the release idempotent per hour by embedding a
+          # slot marker in the release commit message.
+          if git log origin/main --grep="\\[rc-slot:${{ steps.slot.outputs.value }}\\]" -n 1 --format=%H | grep -q .; then
+            echo "already_ran=true" >>"$GITHUB_OUTPUT"
+            exit 0
+          fi
+
+          echo "already_ran=false" >>"$GITHUB_OUTPUT"
 
       - name: Create and push RC tag
+        if: steps.existing.outputs.already_ran != 'true'
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
-          npm version prerelease --preid=rc
+          npm version prerelease --preid=rc -m "release: %s [rc-slot:${{ steps.slot.outputs.value }}]"
           git push origin main --follow-tags