fix(ai): keep a cut answer from a model never seen reasoning

A reply that ran into the token ceiling with unlabelled text was read as
reasoning the server did not separate whenever the ceiling held room for
reasoning, which is every call that carries a level, Off included. A model
that does not reason at all (llama3.1) lost its own truncated answer: the
call failed as reasoning_only, the briefing never reached its JSON-shape
retry, the Coach named the wrong error, the failure counted toward the
bench, and the client learned to ask with "none" for an hour.

Such text is now reasoning only once the endpoint and model have shown that
they reason: a reasoning field or a think tag in an earlier reply, or a
lesson that says so (knownReasoner). A sighting on a request that asked to
reason speaks only for requests that ask too. Otherwise the text is the
answer, cut off, with finishReason "length".

A lone closing </think> now splits a reply only where the chat templates put
it, at the start of a line, or anywhere once the model is known to reason. An
ordinary answer that mentions the tag in a sentence stays whole.

Two expectations encoded the old rule and move: the Coach loop case whose
first reply is unlabelled template-opened text from a model never seen
reasoning, and the Local case that read a never-seen model's cut text as a
thought. The streamed think-block splitting moved into local-reasoning.ts.

Refs #1276

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

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
   "count": 351,
-  "total": 113581,
+  "total": 113530,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 314,
@@ -254,8 +254,8 @@
     "lib/ai/coach/turn/pipeline.ts :: runTurn": 274,
     "lib/ai/coach/turn/reply-guards.ts :: guardReply": 331,
     "lib/ai/codex-client.ts :: parseResponse": 302,
-    "lib/ai/local-client.ts :: generateCompletionStream": 263,
-    "lib/ai/openai-client.ts :: generateCompletion": 276,
+    "lib/ai/local-client.ts :: generateCompletionStream": 222,
+    "lib/ai/openai-client.ts :: generateCompletion": 266,
     "lib/analytics/bp-in-target-fast-path.ts :: computeFromRollups": 203,
     "lib/analytics/compliance/display.ts :: calculateCompliance": 212,
     "lib/analytics/correlations-fast-path.ts :: computeCorrelationHypothesesFastPath": 256,