fix(ai): keep lab values out of every model prompt while Labs is off

The Coach snapshot attached its labs block regardless of the module, a
decision from before Labs could be switched off, while the snapshot's own
prelude says a disabled domain never reaches the model. With Labs off, the
Coach prompt, its get_labs tool, the daily briefing's flagged-marker block,
the biomarker status note, the reaction line to a new panel and the MCP
lab_trend_brief prompt all still read lab values for a model.

Each of those reads now asks the labs module first and reads nothing when it
is off: the Coach tool answers module_disabled, the briefing omits the
block, the status note neither writes nor serves text, the reaction line
gets no value, and the MCP prompt refuses like get_labs and the doctor-visit
prompt already did. The data routes stay as they are. A structural guard
holds every lab-table reader under the model-input paths to the gate or to
a listed reason its rows never reach a model.

src/__tests__/baselines/source-file-lines.json

@@ -68,7 +68,7 @@
     "lib/jobs/insight-pregenerate.ts": 1189,
     "lib/jobs/medication-intake-import.ts": 819,
     "lib/jobs/offhost-backup.ts": 1147,
-    "lib/jobs/reaction-line.ts": 824,
+    "lib/jobs/reaction-line.ts": 803,
     "lib/jobs/reminder/medication-reminder-check.ts": 816,
     "lib/managed-profiles/handover.ts": 986,
     "lib/mcp/record-search.ts": 1467,