feat(import): warn when importing agent history while logged out

Importing (via entire import or the entire enable offer) writes read-only checkpoints to the local entire/checkpoints/v1 store and never syncs on its own — sync happens later via the git pre-push hook, only once logged in. So an import run while logged out succeeds locally but silently never reaches the dashboard, and re-importing after login is a no-op (idempotent on local existence).

Surface this: after a logged-out import that wrote local history, print a notice that it's local-only and to log in before importing so it syncs. No-op when logged in or when nothing was imported. Login detection is local-only (ENTIRE_TOKEN or a current stored context) — no network on the import path. The existing "Imported N turn(s)" line is unchanged.

This is fix #1 (the UX guard) for #1773; the sync-trigger for already-imported, commit-less checkpoints is tracked separately there.