[FEAT] PR-aware restore + branch preview

[haimbj1]

What

• The ledger now stores each session's refs (from its status note). A dead entry keeps its last refs.
• A PR row whose owner session died shows "⟲ Restore · " instead of "Start session" — one click resumes the exact session, context intact. Match: repo-qualified ref, bare #N, then worktree path. Ended-session rows show ref pills.
• preview.sh [port]: run any branch on its own port against a snapshot copy of the real data. The live service stays untouched; Ctrl-C cleans up. This is how UI PRs get seen before merge.

Why

After a reboot the owner could not tell which ended session belonged to which PR, and had no way to see a UI change before merging it.