Pre-existing flaky test: the GSTACK_HOME-overrides-real-config test asserted
the real ~/.gstack/config.yaml does NOT contain "gbrain_sync_mode: full"
after the test. That fails for any user whose real config legitimately has
that key set from prior usage — the test's invariant is "the command did
not modify the real file," not "the real file lacks any specific value."
Switch to mtime + content snapshot: capture both BEFORE running the command,
then verify both are unchanged after. Also add a positive assertion that
the tmpHome config DID get the new key.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>