refactor: remove all strix/ model alias machinery

The Strix proxy / ``strix/`` model namespace is gone. Users now pass
real provider aliases directly (``anthropic/claude-sonnet-4-6``,
``openai/gpt-5.4``, ``gemini/...``, ``openrouter/...``).

Deleted:
- ``STRIX_API_BASE`` constant in ``strix/config/config.py`` (and the
  auto-set api_base branch for ``strix/`` models in ``resolve_llm_config``).
- ``STRIX_MODEL_MAP`` and the ``StrixModelProvider`` /
  ``LitellmAnthropicProvider`` classes from
  ``strix/llm/multi_provider_setup.py``.
- ``is_anthropic_override`` flag on ``AnthropicCachingLitellmModel``
  (only existed because ``strix/<alias>`` resolved to ``openai/<base>``
  on the wire while staying Anthropic underneath; with no proxy, the
  model-name substring check is enough).
- ``startswith("strix/")`` branches in ``cli.py`` / ``main.py`` /
  ``dedupe.py`` and the ``uses_strix_models`` env-validation flag.

The new ``build_multi_provider`` registers a single ``anthropic/``
route that wraps litellm in :class:`AnthropicCachingLitellmModel`
(prompt caching). Every other prefix falls through to the SDK's
built-in routing.

Defaults flipped from ``strix/claude-sonnet-4.6`` →
``anthropic/claude-sonnet-4-6`` in run_config_factory and
agents_graph/tools.py + corresponding tests.

Tests updated:
- ``test_anthropic_cache_wrapper.py``: drop the override-flag tests.
- ``test_multi_provider_setup.py``: rewrite around the new single
  ``_AnthropicCachingProvider`` route.
- ``test_tool_registration_modes.py::test_load_skill_import_...``:
  load_skill no longer fails when there's no live agent instance — it
  echoes the requested skills back with ``success=True``.

Tests: 281/281 passing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

strix/tools/agents_graph/tools.py

@@ -361,7 +361,7 @@ async def create_agent(
         agent_name=name,
         parent_id=parent_id,
         tracer=inner.get("tracer"),
-        model=inner.get("model", "strix/claude-sonnet-4.6"),
+        model=inner.get("model", "anthropic/claude-sonnet-4-6"),
         model_settings=inner.get("model_settings"),
         max_turns=int(inner.get("max_turns", 300)),
         is_whitebox=bool(inner.get("is_whitebox", False)),
@@ -373,7 +373,7 @@ async def create_agent(
     child_run_config = make_run_config(
         sandbox_session=inner.get("sandbox_session"),
         sandbox_client=inner.get("sandbox_client"),
-        model=inner.get("model", "strix/claude-sonnet-4.6"),
+        model=inner.get("model", "anthropic/claude-sonnet-4-6"),
         model_settings_override=inner.get("model_settings"),
     )