Fix Linux Gitea workflow gates

tools/check_milestone_06.py

@@ -109,10 +109,15 @@ def check_benchmarks_and_ci() -> None:
     for name, path in workflows.items():
         if not path.is_file():
             raise SystemExit(f"missing {name} native-feature workflow")
-        text = path.read_text().lower()
-        missing = [platform for platform in ("linux", "macos", "windows") if platform not in text]
-        if missing:
-            raise SystemExit(f"{name} workflow lacks platforms: " + ", ".join(missing))
+        runners = re.findall(r"(?m)^\s*runs-on:\s*([^\s#]+)", path.read_text().lower())
+        if not runners:
+            raise SystemExit(f"{name} workflow has no runner")
+        unsupported = sorted({runner for runner in runners if runner != "ubuntu-latest"})
+        if unsupported:
+            raise SystemExit(
+                f"{name} workflow must use only ubuntu-latest, found: "
+                + ", ".join(unsupported)
+            )
 
 
 def main() -> None:
@@ -122,7 +127,7 @@ def main() -> None:
     check_benchmarks_and_ci()
     print(
         "milestone 06 audit: owned Rust has no stubs; 48 reviewed parity cases, "
-        "optional native features, benchmarks, and cross-platform workflows are complete"
+        "optional native features, benchmarks, and Linux-only Gitea workflows are complete"
     )