feat(import): restore soft-deleted conversations from the session picker

Deleting a conversation in codeg only stamps `deleted_at` — the row and the
agent's own session file both stay on disk — so a deleted session that the
import picker rediscovers has a meaningful answer beyond "skip": bring it back.

The picker now offers `deleted` rows behind an explicit "include deleted"
switch (off by default). Importing one un-deletes the ORIGINAL row rather than
inserting a duplicate, so its id, history, bound tabs and delegation children
all survive; it lands in the group's folder (the one `add_folder` just made
live and open, since the row's own folder may since have been removed) and on
`pending_review`, so the sidebar's default "hide completed" filter cannot make
a restore look like it failed.

Restoring is opt-in per call via `DeletedPolicy`: only the picker — where every
item is a session the user checked while it was badged "deleted" — passes
`Restore`. The legacy whole-folder sweep and the scan's drive-by refresh stay
on `Skip`, so neither can resurrect a conversation the user deliberately
deleted. On the client, select-all, the folder tri-state and the sidebar's
focus-path preselect all keep ignoring deleted rows until the switch is on, and
turning it back off un-stages whatever it staged.