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/config/config.py

@@ -32,13 +32,10 @@ class Config:
 
     # Tool & Feature Configuration
     perplexity_api_key = None
-    strix_disable_browser = "false"
 
     # Runtime Configuration
     strix_image = "ghcr.io/usestrix/strix-sandbox:0.1.13"
     strix_runtime_backend = "docker"
-    strix_sandbox_execution_timeout = "120"
-    strix_sandbox_connect_timeout = "10"
 
     # Telemetry
     strix_telemetry = "1"