The install dialog gated the package path on the agent install succeeding in
the same click (`agentInstalled && desktopHalfFromPackage`). A package already
on disk answers "Plugin '<name>' already exists. Use force reinstall" without
Force, so the retry fell through to installDesktopPlugin and cloned
desktop-plugins/<git-name>/ beside the package copy the app had already
materialised as desktop-plugins/<manifest-name>/. Two folders, one plugin id:
the on-disk duplicate from #100412 (since e1a6679399 an error row instead of
a competing live instance, but still created by the install flow).
The package path now applies whenever the repo is a unified package, the
backend is local and the Agent box is ticked, whatever the agent install
returned. reconcileDesktopPlugins() is idempotent, so a retry touches
nothing; the desktop success toast is raised only when the agent half landed
or a copy was actually materialised, so a refused install shows the agent
error alone. A failed fresh install no longer leaves a standalone desktop
clone behind for the next successful install to duplicate.
Remote backends keep the separate clone (their plugins/ folder is not
readable from this machine); the Desktop-UI-only tick is unchanged.
Fixes#100412 (install-time half; runtime half landed in #118902).
Tests: two invariants in plugin-install-modal.test.tsx, the first red on
main (installDesktopPlugin called on the refused retry), the second guarding
the remote-backend clone.
(cherry picked from commit 9fbd3a9d8328ea452a4149252a35a0e6f43982e6)
035171c7c9