feat(P3): mind/mouth split — separate voice model for the final reply (seam, default off)

The mind (chat backend/model) decides, reasons, and runs tools → a draft; the mouth
re-voices that draft in her character. Default: no mouth configured → the mind's
draft IS the reply, bit-for-bit the old behavior (and old streaming path untouched).

- config: MOUTH_BACKEND / MOUTH_MODEL. The slot for an eventual fine-tuned voice.
- chat: _mind_loop (tool/generation loop, non-stream, returns draft + tools_run),
  _voice_pass / mind.voice_messages (re-voice the draft, keep every fact/number),
  _mouth_target (active only when configured AND != mind). respond + respond_stream
  branch: mouth off = stream the mind directly (unchanged); mouth on = mind decides
  + runs tools, then the mouth streams the re-voiced reply. Falls back to the draft
  on any mouth failure (chat never breaks).
- Key payoff: the mouth needs no tool support (the mind handles tools), so it can be
  a non-tool character model (Dolphin / Claude / fine-tune). Makes the fine-tune
  easy: teach a small model to *sound* like Lyra, not to be smart.
- tests: mouth target on/off, voice_messages shape, voice_pass revoice+fallback.
  Suite 96 green, ruff clean.

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

lyra/mind.py

@@ -290,6 +290,25 @@ def _deliberate_part(ctx: TurnContext) -> TurnContext:
 PIPELINE = (_perceive, _route, _compose, _deliberate_part)
 
 
+# --- mouth (the voice pass: re-render the mind's draft in her character) -----
+
+_VOICE_NOTE = (
+    "↑ That was you working the answer out — a draft Brian has NOT seen. Now say it to him "
+    "in your own voice: warm, direct, specific, in character, opinionated. Keep every fact, "
+    "number, name, and decision exactly as in the draft — change only the wording so it sounds "
+    "like you, not a generic assistant. No preamble, no meta, no 'here's a friendlier version' "
+    "— just your actual message to Brian."
+)
+
+
+def voice_messages(messages: list[Message], draft: str) -> list[Message]:
+    """Prompt for the mouth model: the full turn context + the mind's draft to re-voice."""
+    return messages + [
+        {"role": "assistant", "content": draft},
+        {"role": "system", "content": _VOICE_NOTE},
+    ]
+
+
 def assemble(session_id: str, user_msg: str, backend: Backend,
              model: str | None = None) -> TurnContext:
     """Run the parts over a fresh TurnContext and return it ready for `chat` to speak."""