fix(chat): read a task's output in a file tab instead of the OS opener

Widening `opener:allow-open-path` to the OS temp roots bought a real
privilege — hand any file under /tmp to whatever the system decides to
run — to read a log, and it still didn't work: the button also fires
`shell.open` on a `.output` extension no application claims.

Open it the way a file named in the transcript opens (`reply-artifacts`):
`openFilePreview` takes an absolute path anywhere, reads through codeg's
own backend, and renders in a file tab. No capability scope to widen when
an adapter picks a new directory, no guessing at a handler, and it works
in web — so the button drops its `isLocalDesktop()` gate too. The scope
goes back to `$HOME/**`.

Also drops the "shown above" tag. It answered a question nobody was
asking, in the row's most valuable space — right beside the task name.
`show_in_transcript` stays on the wire; nothing reads it now.