feat(vendor_capabilities): implement registry with initial 22-entry population

Green phase: src/vendor_capabilities.py now exists and all 3 Red-phase
tests in tests/test_vendor_capabilities.py pass.

Implementation:
- VendorCapabilities frozen dataclass with 12 fields (vendor, model, vision,
  tool_calling, caching, streaming, model_discovery, context_window,
  cost_tracking, cost_input_per_mtok, cost_output_per_mtok, notes)
- Module-level _REGISTRY dict keyed by (vendor, model)
- register() inserts/overwrites entries
- get_capabilities() returns specific entry if present, else vendor '*'
  default, else raises KeyError with 'No capabilities registered' message
- list_models_for_vendor() returns sorted model names for a vendor
  (excludes '*' wildcard)

Initial population (22 entries at module load):
- 1 minimax wildcard (cost: 0.20/0.20 per Mtok)
- 4 grok (1 wildcard + 3 models; grok-2-vision has vision=True)
- 9 llama (1 wildcard + 8 models; 11b/90b vision variants have vision=True)
- 8 qwen (1 wildcard + 7 models; qwen-vl-plus/max have vision=True;
  qwen-audio has notes='Text-only in v1; audio input deferred')

The plan's Task 1.3 listed 22 entries but included one impossible entry
(vendor='minimax', model='grok-2-latest'). Omitted; 21 entries shipped.

Test fix: test_fallback_to_vendor_default previously used model name
'llama-3.3-70b-specdec' which IS in the registry, so the specific entry
was returned (with default cost_tracking=True), not the wildcard. Fixed
by changing to 'llama-3.3-future-unregistered' (not in registry, so
fallback fires correctly).

tests/test_vendor_capabilities.py

@@ -31,7 +31,7 @@ def test_fallback_to_vendor_default():
   cost_tracking=False
  )
  register(caps)
- retrieved = get_capabilities('llama', 'llama-3.3-70b-specdec')
+ retrieved = get_capabilities('llama', 'llama-3.3-future-unregistered')
  assert retrieved.context_window == 131072
  assert retrieved.cost_tracking is False