refactor(P1): extract the turn pipeline into lyra/mind.py (behavior-preserving)

First step of the cognition control plane (docs/COGNITION.md). The chat turn is now
an explicit society of parts over a shared TurnContext blackboard:
  perceive (stub) -> route (session mode) -> compose (tiered prompt) -> deliberate.

- lyra/mind.py (new): TurnContext + the pipeline + assemble(); moved build_messages
  and the deliberation helpers here (the assembly belongs in the control plane).
- lyra/chat.py: slimmed to "speak + persist" — calls mind.assemble(), runs the
  tool/generation loop, persists. No behavior change (same prompt, same output).
- tests: point test_time/test_chat at mind; add an assemble() structure test;
  make test_chat/test_tools hermetic (CHAT_DELIBERATE off so respond() doesn't make
  a real LLM call). Suite 86 green in ~5s, ruff clean, no import cycle.

This is the frame; perceive/route/learn get filled in next phases — each opt-in.

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

tests/test_tools.py

@@ -9,6 +9,7 @@ import pytest
 @pytest.fixture
 def lyra(tmp_path, monkeypatch):
     monkeypatch.setenv("LYRA_DB_PATH", str(tmp_path / "test.db"))
+    monkeypatch.setenv("CHAT_DELIBERATE", "false")  # don't make a real LLM call in respond()
     from lyra import llm
     monkeypatch.setattr(llm, "embed", lambda texts: [[0.1, 0.2, 0.3] for _ in texts])
     import lyra.memory as memory