fix: address audit findings — SDK plumbing, TUI bus, dead code

Critical fixes:

- ``StrixOrchestrationHooks.on_agent_start`` now finds the
  ``CaidoCapability`` via ``ctx.context['caido_capability']`` instead
  of ``agent.capabilities`` (we use plain ``Agent``, not
  ``SandboxAgent``, so the latter never existed). The session
  manager's bundle already exposes the capability; ``run_strix_scan``
  threads it through ``make_agent_context`` and ``create_agent``
  forwards it to children.

- ``run_strix_scan`` registers the ``StrixTracingProcessor`` with the
  SDK's tracing provider via ``add_trace_processor`` so SDK trace
  spans hit ``run_dir/events.jsonl`` (was previously a parallel stream
  the SDK ignored).

- ``on_llm_end`` now writes to ``Tracer.record_llm_usage`` in
  addition to ``bus.record_usage`` so the CLI/TUI stats panel sees
  real numbers instead of zeros.

- ``run_strix_scan`` accepts an externally-built ``AgentMessageBus``
  + an explicit ``model`` arg. The TUI pre-creates the bus so its
  stop and chat-input handlers can submit ``bus.send`` /
  ``bus.cancel_descendants`` coroutines onto the scan thread's loop
  via ``asyncio.run_coroutine_threadsafe`` — replacing the
  TODO-stub no-ops.

- ``model`` config now propagates root → context → child agents in
  ``create_agent`` (was hardcoded fallback).

Dead-code removal:

- Deleted the ``load_skill`` tool entirely (host module, sandbox
  module, TUI renderer, tests). The legacy implementation reached
  into a global ``_agent_instances`` registry that no longer exists;
  the post-migration stub returned ``success=True`` without
  injecting anything — pure theater. Skills are still preloaded via
  the system prompt at scan-bring-up.

- Dropped ``tenacity`` and ``xmltodict`` from
  ``[project.dependencies]`` — neither is imported anywhere
  post-migration.

- Stripped the system prompt's "use the load_skill tool" lines.

Tests: 278/278 passing. Removed two ``load_skill`` test cases and a
``test_tool_registration_modes::test_load_skill_import_...`` assertion
that exercised the deleted module.

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

strix/run_config_factory.py

@@ -171,6 +171,7 @@ def make_agent_context(
     run_id: str | None = None,
     sandbox_client: Any | None = None,
     agent_factory: Any | None = None,
+    caido_capability: Any | None = None,
 ) -> dict[str, Any]:
     """Build the per-agent ``context`` dict passed to ``Runner.run(context=...)``.
 
@@ -190,6 +191,7 @@ def make_agent_context(
         "sandbox_token": sandbox_token,
         "tool_server_host_port": tool_server_host_port,
         "caido_host_port": caido_host_port,
+        "caido_capability": caido_capability,
         "agent_id": agent_id,
         "agent_name": agent_name,
         "parent_id": parent_id,