Fix Hugh's Gemini heartbeat compliance

Pin model from auto to gemini-2.5-pro, add explicit checkout
and X-Paperclip-Run-Id headers, and add required output gates
for steps 3-5 so Gemini can't silently skip them.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

engineering/hugh/CONFIG.md

@@ -34,7 +34,7 @@
     "GITHUB_APP_ID_HUGH": { "type": "plain", "value": "3034857" },
     "GITHUB_PEM_PATH_HUGH": { "type": "plain", "value": "/paperclip/secrets/github-pems/hugh.pem" }
   },
-  "model": "auto",
+  "model": "gemini-2.5-pro",
   "graceSec": 15,
   "timeoutSec": 0,
   "maxTurnsPerRun": 80,