chore: final cleanup — drop `STRIX_SANDBOX_MODE / strix_disable_browser` / runtime docstring

Tail end of the sandbox-tools migration:
- Drop ``ENV STRIX_SANDBOX_MODE=true`` and ``ENV PYTHONPATH=/app`` from
  the Dockerfile — both only mattered for the now-deleted in-container
  tool server (the legacy ``register_tool`` registry gated on the env
  var, and the entrypoint set ``PYTHONPATH`` so it could ``-m
  strix.runtime.tool_server``).
- Drop ``strix_disable_browser`` from the Config defaults — the legacy
  registry used it to skip ``browser_action`` registration; agent-browser
  is unconditional now.
- Strip the ``tool_server.py`` blurb from ``strix/runtime/__init__.py``.

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

strix/runtime/__init__.py

@@ -5,9 +5,4 @@
   ``NET_ADMIN`` / ``NET_RAW`` capabilities and ``host.docker.internal``
   extra-hosts, used by the per-scan session manager
   (:mod:`strix.sandbox.session_manager`).
-
-- ``tool_server.py`` — FastAPI server that runs inside the sandbox
-  container. Sandbox-bound tools (browser, terminal, python, file_edit,
-  proxy) POST here from the host via
-  :func:`strix.tools._sandbox_dispatch.post_to_sandbox`.
 """