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>

containers/Dockerfile

@@ -192,8 +192,6 @@ RUN ARCH=$(uname -m) && \
     rm caido-cli.tar.gz && \
     mv caido-cli /usr/local/bin/
 
-ENV STRIX_SANDBOX_MODE=true
-ENV PYTHONPATH=/app
 ENV REQUESTS_CA_BUNDLE=/etc/ssl/certs/ca-certificates.crt
 ENV SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt