fix(builtin_tools/temporal_workflow): collapse dead if/else in _platform_url()

RC 835 (infra-lead): the if/else that checked for Docker and set the
PLATFORM_URL default had both branches return the same value. Collapsed
to a single return statement with a docstring explaining why the legacy
non-Docker branch is removed.

This completes Path B: all 6 PLATFORM_URL sites in this PR now have
clean, non-dead default logic.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workspace/builtin_tools/temporal_workflow.py

@@ -56,17 +56,15 @@ logger = logging.getLogger(__name__)
 
 
 def _platform_url() -> str:
-    """Return the platform URL, defaulting to host.docker.internal when running
-    inside a Docker container (where localhost refers to the container, not the
-    host).  External callers can always override via the PLATFORM_URL env var.
+    """Return the platform URL, defaulting to host.docker.internal.
 
-    Note: both branches now use the same default — the platform API is only
-    reachable via host.docker.internal from within a workspace container,
-    regardless of how the container was started. The if/else structure is
-    preserved for future extensibility.
+    The workspace runtime always runs inside a Docker container, so
+    ``localhost`` refers to the container itself, not the platform host.
+    The platform API is only reachable via ``host.docker.internal`` from
+    within a workspace container, regardless of how the container was started.
+    The legacy non-Docker branch is removed (it would have returned
+    ``localhost:8080`` which is unreachable from inside the container).
     """
-    if os.path.exists("/.dockerenv") or os.environ.get("DOCKER_VERSION"):
-        return os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
     return os.environ.get("PLATFORM_URL", "http://host.docker.internal:8080")
 
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