The rule the preview-tool bug broke, written down so the next GUI-adjacent tool
does not rediscover it: the client and the backend are separate machines, so
"was this process spawned by Electron?" cannot answer "is a GUI watching?".
Names the working pattern (toolset gates the surface, check_fn answers only
reachability or user opt-in), the process-wide check_fn TTL cache that makes it
the wrong home for a per-session answer, and the test that would have caught
it — assert the GUI session gets the tool with the env var absent.
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