refactor(box): clean up sandbox subsystem code quality and efficiency

- Fix O(n²) stderr trimming in runtime.py with running length tracker
  - Remove dead code: RESERVED_CONTAINER_PATHS, _subprocess_wait_task,
    unused config_hash computation, unused imports
  - Deduplicate connection callback in BoxRuntimeConnector, parse URL once
  - Use enum comparison instead of stringly-typed spec.network.value check
  - Replace manual _result_to_dict/_session_to_dict with model_dump()
  - Cache NativeToolLoader tool definition and sandbox system guidance
  - Extract _is_path_under() helper to eliminate duplicated path checks
  - Import SANDBOX_EXEC_TOOL_NAME from native.py instead of redefining
  - Add JSON startswith guard in logging_utils to skip futile json.loads
  - Fix ruff lint errors (F401 unused imports, F841 unused variables)

src/langbot/pkg/box/security.py

@@ -20,13 +20,6 @@ BLOCKED_HOST_PATHS = frozenset({
     '/var/run/podman',
 })
 
-RESERVED_CONTAINER_PATHS = frozenset({
-    '/workspace',
-    '/tmp',
-    '/var/tmp',
-    '/run',
-})
-
 
 def validate_sandbox_security(spec: BoxSpec) -> None:
     """Validate that a BoxSpec does not request dangerous container config.