refactor(ai_client): move PROVIDERS to src/ai_client.py; re-export via models.__getattr__

Phase 2 tasks 2.1 + 2.2 + 2.3a of the follow-up track.

PROVIDERS now lives in src/ai_client.py:56 (the canonical home for
AI-client-related constants per the HARD RULE on src/ files). The
list includes all 8 vendors: gemini, anthropic, gemini_cli,
deepseek, minimax, qwen, grok, llama.

Backward compat: src/models.py:PROVIDERS is exposed via a module-
level __getattr__ (PEP 562) that lazy-imports from src.ai_client.
The lazy approach was needed because src.ai_client imports
ToolPreset/BiasProfile/Tool from src.models at line 50, so a
top-level 'from src.ai_client import PROVIDERS' in models.py
would deadlock. Adding a branch to the existing __getattr__
in models.py (which also handles pydantic class factories) is
the surgical fix.

tests/test_provider_curation.py was stale (expected 5 providers
from before Qwen/Grok/Llama were added). Updated to 8.

New test: tests/test_providers_source_of_truth.py asserts:
- src.ai_client.PROVIDERS exists and matches the 8-provider list
- src.models.PROVIDERS still works (re-export)
- Both modules reference the SAME object (no drift)

Green confirmed: 4 provider tests pass.

src/models.py

@@ -53,7 +53,14 @@ from src.paths      import get_config_path
 
 #region: Constants
 
-PROVIDERS: List[str] = ["gemini", "anthropic", "gemini_cli", "deepseek", "minimax", "qwen", "grok", "llama"]
+# PROVIDERS is the source of truth in src/ai_client.py (per the
+# follow-up track's Naming Convention HARD RULE). Lazy-loaded
+# via the __getattr__ defined later in this module to break the
+# circular import (src.ai_client imports ToolPreset/BiasProfile/
+# Tool from this module at line 50, so a top-level 'from
+# src.ai_client import PROVIDERS' here would deadlock). The
+# audit script scripts/audit_providers_source_of_truth.py
+# verifies PROVIDERS is declared in src/ai_client.py and not here.
 
 AGENT_TOOL_NAMES: List[str] = [
  "run_powershell",
@@ -251,6 +258,9 @@ _PYDANTIC_CLASS_FACTORIES: dict[str, callable] = {
 }
 
 def __getattr__(name: str) -> Any:
+ if name == "PROVIDERS":
+  from src.ai_client import PROVIDERS as _PROVIDERS
+  return _PROVIDERS
  if name in _PYDANTIC_CLASS_FACTORIES:
   cls = _PYDANTIC_CLASS_FACTORIES[name]()
   globals()[name] = cls