ci: bump continuous-synth-e2e cadence 3→6 fires/hour, all clean slots

Change cron from '10,30,50' (3 fires/hour) to '2,12,22,32,42,52'
(6 fires/hour). All new slots are 1-3 min away from any other
cron, avoiding both the cf-sweep collisions (:15, :45) and the
:30 heavy slot (canary-staging /30, sweep-aws-secrets,
sweep-stale-e2e-orgs every :15).

Why: empirically 2026-05-04 the canary fired only once per hour
on the 10,30,50 schedule (see #2726). Bumping fires-per-hour
gives more chances to land a survived fire under GH's load-
related drop ratio, and keeping all slots in clean lanes
minimizes the per-fire drop probability.

At empirically-observed ~67% drop ratio, 6 attempts/hour yields
~2 effective fires = ~30 min cadence; closer to the 20-min
target than the current shape and provides a real degradation
alarm if drops get worse.

Cost: ~$0.50/day → ~$1/day. Negligible.

Closes #2726.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/continuous-synth-e2e.yml

@@ -32,20 +32,30 @@ name: Continuous synthetic E2E (staging)
 
 on:
   schedule:
-    # Every 20 minutes, on :10 :30 :50. Two constraints:
+    # Every 10 minutes, on :02 :12 :22 :32 :42 :52. Three constraints:
     #   1. Stay off the top-of-hour. GitHub Actions scheduler drops
     #      :00 firings under high load (own docs:
     #      https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule).
-    #      Empirical 2026-05-03: cron was '0,20,40 * * * *' but actual
-    #      firings landed at :08, :03, :01, :03 with :20 + :40 silently
-    #      dropped — only the :00-region run survived. Detection
-    #      latency degraded from claimed 20 min to actual ~60 min.
-    #      :10/:30/:50 sit far enough from :00 that GH-load skips
-    #      stop dropping us.
+    #      Prior history: cron was '0,20,40' (2026-05-02) — only :00
+    #      ever survived. Bumped to '10,30,50' (2026-05-03) on the
+    #      theory that further-from-:00 wins. Empirically 2026-05-04
+    #      that ALSO dropped to ~60 min effective cadence (only ~1
+    #      schedule fire per hour — see molecule-core#2726). Detection
+    #      latency was claimed 20 min, actual 60 min.
     #   2. Avoid colliding with the existing :15 sweep-cf-orphans
     #      and :45 sweep-cf-tunnels — both hit the CF API and we
     #      don't want to fight for rate-limit tokens.
-    - cron: '10,30,50 * * * *'
+    #   3. Avoid the :30 heavy slot (canary-staging /30, sweep-aws-
+    #      secrets, sweep-stale-e2e-orgs every :15) — multiple
+    #      overlapping cron registrations on the same minute is part
+    #      of what GH drops under load.
+    # Solution: bump fires-per-hour 3 → 6 AND keep all slots in clean
+    # lanes (1-3 min away from any other cron). Even with empirically-
+    # observed ~67% GH drop ratio, 6 attempts/hour yields ~2 effective
+    # fires = ~30 min cadence; closer to the 20-min target than the
+    # current shape and provides a real degradation alarm if drops
+    # get worse.
+    - cron: '2,12,22,32,42,52 * * * *'
   workflow_dispatch:
     inputs:
       runtime: