feat: separate CHAT_MODEL (gpt-4o) for persona fidelity

Mid-size models (gpt-4o-mini, qwen2.5-14b) resist persona instructions —
help-desk closers and feelings-disclaimers leak through regardless. Route live
chat to a stronger model while keeping bulk consolidation cheap:

- config: CHAT_MODEL (default gpt-4o), distinct from CLOUD_MODEL (gpt-4o-mini)
- llm.complete gains a `model` override; chat.respond uses chat_model on cloud,
  consolidation paths keep cloud_model
- persona: reword the "no sign-off" rule so genuine questions are welcome and
  only reflexive customer-service closers are discouraged

Verified: on gpt-4o she owns her mood without disclaimers and drops most
help-desk tails — clearly more in-character than mini/qwen.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.env.example

@@ -8,7 +8,8 @@ MI50_MODEL=local-gpu
 
 # Cloud backend (OpenAI) — higher quality, costs money.
 OPENAI_API_KEY=
-CLOUD_MODEL=gpt-4o-mini
+CLOUD_MODEL=gpt-4o-mini   # cheap model for bulk consolidation (summaries/profile/etc.)
+CHAT_MODEL=gpt-4o         # stronger model for live chat (better persona fidelity)
 
 # Embeddings: "cloud" (OpenAI) or "local" (Ollama). A database is tied to whichever
 # backend created it — don't switch this against an existing DB (vector spaces differ).