refactor(ai_client): merge vendor_capabilities into ai_client; git rm src/vendor_capabilities.py

Per spec FR2 + Phase 2.1: VendorCapabilities + register + get_capabilities +
list_models_for_vendor + the ~40 vendor registrations move into ai_client.py
as a region block. Renamed internal _REGISTRY to _VENDOR_REGISTRY to avoid
collision with mcp_tool_specs._REGISTRY.

Importers (in src/) updated:
- src/ai_client.py: removed top-level import; removed 4 local imports of
  list_models_for_vendor/get_capabilities (symbol now in module namespace)
- src/app_controller.py: 2 sites updated to 'from src.ai_client import get_capabilities'
- src/gui_2.py: 1 site updated to 'from src.ai_client import VendorCapabilities, get_capabilities'

Tests updated:
- 8 test_*.py files: changed 'from src.vendor_capabilities import' to
  'from src.ai_client import'
- tests/test_vendor_capabilities.py: _clean_registry fixture updated to
  reference src.ai_client._VENDOR_REGISTRY (was src.vendor_capabilities._REGISTRY)

Verification: 157 tests pass across the affected files (vendor_capabilities,
ai_client_tool_loop variants, openai_compatible, command_palette,
diff_viewer, patch_modal, app_controller_result, app_controller_sigint,
handle_reset_session, ai_loop_regressions, grok/llama/minimax provider tests).

tests/test_vendor_capabilities.py

@@ -1,13 +1,13 @@
 import pytest
-from src.vendor_capabilities import VendorCapabilities, get_capabilities, register
+from src.ai_client import VendorCapabilities, get_capabilities, register
 
 @pytest.fixture(autouse=True)
 def _clean_registry():
- import src.vendor_capabilities
- snapshot = src.vendor_capabilities._REGISTRY.copy()
+ import src.ai_client as _ai
+ snapshot = _ai._VENDOR_REGISTRY.copy()
  yield
- src.vendor_capabilities._REGISTRY.clear()
- src.vendor_capabilities._REGISTRY.update(snapshot)
+ _ai._VENDOR_REGISTRY.clear()
+ _ai._VENDOR_REGISTRY.update(snapshot)
 
 def test_registry_lookup_known_model():
  caps = VendorCapabilities(
@@ -217,6 +217,6 @@ def test_v2_capability_badge_helper_skips_disabled_fields() -> None:
  a live context, but we can verify the helper is a no-op on
  the no-cap case.)"""
  from src.gui_2 import _render_v2_capability_badges
- from src.vendor_capabilities import VendorCapabilities
+ from src.ai_client import VendorCapabilities
  empty_caps = VendorCapabilities(vendor='test', model='empty')
  _render_v2_capability_badges(empty_caps)