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/ai_client.py

@@ -51,8 +51,11 @@ from src.models       import ToolPreset, BiasProfile, Tool
 from src.paths        import get_credentials_path
 from src.tool_bias    import ToolBiasEngine
 from src.tool_presets import ToolPresetManager
+from src.tool_presets import ToolPresetManager
 
+PROVIDERS: List[str] = ["gemini", "anthropic", "gemini_cli", "deepseek", "minimax", "qwen", "grok", "llama"]
 
+# _require_warmed lives
 # _require_warmed lives in src/module_loader.py to avoid duplicating the
 # lookup logic across files that need heavy modules. Re-exported here so
 # existing call sites and the T3.1 test (which asserts