refactor(ai): hoist Provider core to shared cmd_chat/ai/ (P1)

Move providers.py + profiles.py from cmd_chat/agent/ to a shared
cmd_chat/ai/ package so the operator bridge and the /ai chat agent can
consume one model-agnostic Provider core (groundwork for harness-mode
operators). Pure refactor — no behaviour change.

- cmd_chat/ai/{providers,profiles}.py: the canonical modules (moved verbatim)
- cmd_chat/ai/__init__.py: re-exports the public API
- cmd_chat/agent/{providers,profiles}.py: thin back-compat shims re-exporting
  from cmd_chat.ai (keeps `from cmd_chat.agent.providers import …` working,
  e.g. hh/scripts/bench-native-harness.py)
- internal agent consumers (memory/bridge/__main__/__init__) point at cmd_chat.ai

125 tests pass; shim identity verified (re-exports are the same objects).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd_chat/agent/bridge.py

@@ -22,7 +22,7 @@ import websockets
 
 from ..client.client import Client
 from .memory import MemoryIndex
-from .providers import Msg, Provider, ToolsUnsupported
+from cmd_chat.ai.providers import Msg, Provider, ToolsUnsupported
 
 DEFAULT_SYSTEM = (
     "You are {name}, a helpful AI participant in an encrypted terminal chat "