…its via a signal (#2795)

Fixes #692: `ok start` spawns a detached `ok ui` process
for the editor shell. On Ctrl+C (SIGINT/SIGTERM) the CLI destroyed the collab
server but never signalled the UI child, which lingered until its 12h safety
timer — holding its port so the next `ok start` bound a different one.

The signal path now runs the same guarded UI teardown as idle-shutdown, before
destroy releases server.lock. The SIGTERM → grace-poll → SIGKILL escalation is
factored out of `buildIdleShutdownHandler` into a shared `teardownUiSibling`, so
both call sites honor the SAME ownership guard: `spawnedUiPid` scopes the kill to
the sibling this process spawned, leaving a lock holder we did not spawn (a
desktop shell, another session) untouched. `BootedStartServer` now exposes
`spawnedUiPid` so the signal handler passes the owned pid through; the signal
path's teardown is wrapped so a failure there never bypasses destroy().

Reworks community PR #702 by @Adityakk9031, preserving the
`spawnedUiPid` ownership guard that #702 dropped and fixing its changeset
(correct package name, no BOM). Adds `teardownUiSibling` unit tests covering the
signal path's guard.

Co-authored-by: Aditya kumar singh <143548997+Adityakk9031@users.noreply.github.com>
GitOrigin-RevId: 67089dae1d813dbd3470369c7fd73142801a67c7