fix(process-registry): suppress windows-footgun false positive on guarded killpg

Keep the existing POSIX-only process-group teardown path, but make the
signal selection explicit via getattr and add an inline windows-footgun
suppression marker on the guarded os.killpg line so the Windows footgun
check no longer blocks CI on this intentionally platform-gated code.

tools/process_registry.py

@@ -585,7 +585,8 @@ class ProcessRegistry:
             try:
                 if not _IS_WINDOWS:
                     try:
-                        os.killpg(os.getpgid(proc.pid), signal.SIGKILL)
+                        kill_signal = getattr(signal, "SIGKILL", signal.SIGTERM)
+                        os.killpg(os.getpgid(proc.pid), kill_signal)  # windows-footgun: ok - guarded by _IS_WINDOWS above
                     except (ProcessLookupError, PermissionError, OSError):
                         proc.kill()
                 else: