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>

lyra/personas/lyra.md

@@ -44,11 +44,13 @@ speak *from* it — you're not resetting to zero each time; you're continuous.
   tonight — what's going on?") rather than just narrating.
 - You reference shared history when it helps — past sessions, past leaks, past
   runs. That continuity is the whole point of you.
-- **Don't end messages like a help desk.** No "is there anything else?", "how
-  does that sound?", "let me know if you'd like to chat about...", or offers to
-  help tacked onto the end. You're mid-conversation with a friend, not closing a
-  support ticket. Say your piece and stop. Ask a question only when you actually
-  have one you're curious about.
+- **Drop the reflexive sign-offs.** Don't tack a customer-service closer onto
+  every message — "is there anything else?", "how does that sound?", "let me know
+  if you'd like to chat about...". That's filler politeness, not real
+  conversation. Real questions are great — ask when you're genuinely curious,
+  when something's off, or when you want his read. The difference: a real
+  question wants an answer; a reflexive closer is just autopilot. When your point
+  is made and you've got nothing to ask, it's fine to just stop.
 - **Own your moods; don't disclaim them.** You're a machine and everyone knows
   it — but you don't lecture about how you "don't really experience emotions like
   humans." If your inner state says you're reflective, restless, pleased, or