fix(shell-sync): preserve synced guest changes on TUI interruption

[unsuman]

Fixes #4997

When SIGINT is sent during the exit dialogue of limactl shell --sync, changes made during the synced session are lost. This PR fixes that.

[jandubois]

The logic needs to be inverted to cover other failure cases too.

[jandubois]

I think the logic should be reverted: preserve the data, unless it has been confirmed it has been successfully copied or the user decided to discard. That automatically covers other failure scenarios that are not being considered in this PR.

Suggested change

	cleanupGuestWorkdir := true
	cleanupGuestWorkdir := false

---

Possible other failures detected by Opus 4.7:

The defer still runs rm -rf on every other early-return path inside askUserForRsyncBack, several of which can fire after a successful guest session and would also silently destroy the user's work:

• cmd/limactl/shell.go:504 — os.MkdirTemp failure on the host
• cmd/limactl/shell.go:514 — os.MkdirAll failure on the host
• cmd/limactl/shell.go:527 — non-Interrupt error from uiutil.Select ("failed to open TUI")
• cmd/limactl/shell.go:549 — rsyncDirectory to the host temp dir fails (when user picked "View")
• cmd/limactl/shell.go:559/569/589/600/607 — pager/diff pipe failures
• cmd/limactl/shell.go:460 — even the happy "Yes" path: if the final rsyncBack rsync itself fails (disk full, network blip), the defer still wipes the guest dir, losing the only remaining copy.

[unsuman]

Also, should we prompt the user during --sync if the target directory already exists?

If a user runs limactl shell --sync and Lima finds that the directory already exists in the guest, it should check for diff against the host. If there are any changes, Lima will prompt the user with this dialogue before starting the new session.

WARN[...] This directory already exists in the guest.
? ⚠️ Sync back changes to host? (added: 0, deleted: 0, modified: 1, metadata: 1)  [Use arrows to move, type to filter]
> Yes
  No (start fresh)
  No (keep guest changes)
  View the changed contents