refactor: lift hardcoded model default + fix stale `is_whitebox` docstring

``"anthropic/claude-sonnet-4-6"`` was duplicated as a kwarg default in
5 places (``run_strix_scan``, ``make_run_config``, ``make_agent_context``,
and twice in ``agents_graph.create_agent``'s ``inner.get(..., default)``
calls). The default was actually dead code: ``validate_environment``
requires ``STRIX_LLM`` to be set before any scan starts, and the CLI/TUI
callers don't pass ``model=`` themselves.

Replaced with a single resolution in ``run_strix_scan``:

    resolved_model = model or load_settings().llm.model
    if not resolved_model:
        raise RuntimeError("No LLM model configured. ...")

then propagated explicitly to ``make_agent_context`` and
``make_run_config``. Both lose their string defaults — ``model`` is now
a required kwarg. The graph tool's ``inner.get("model", "...")`` is
``inner["model"]``: the parent context guarantees it's set.

Drive-by: ``run_strix_scan`` docstring still listed ``is_whitebox`` as
a ``scan_config`` key — stale since ``1e641e5`` derived it from
``targets`` instead. Updated.

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

strix/tools/agents_graph/tools.py

@@ -446,7 +446,7 @@ async def create_agent(
         agent_id=child_id,
         parent_id=parent_id,
         tracer=inner.get("tracer"),
-        model=inner.get("model", "anthropic/claude-sonnet-4-6"),
+        model=inner["model"],
         model_settings=inner.get("model_settings"),
         max_turns=int(inner.get("max_turns", 300)),
         is_whitebox=bool(inner.get("is_whitebox", False)),
@@ -458,7 +458,7 @@ 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", "anthropic/claude-sonnet-4-6"),
+        model=inner["model"],
         model_settings_override=inner.get("model_settings"),
     )