feat(session): surface imported sessions as read-only in list/info

Imported sessions are read-only (not resumable/rewindable). Label them so users don't mistake unsupported writes for breakage:

• session list card shows "imported (read-only)"
• session info text adds a read-only note; --json/list --json expose kind + read_only
• resume picker explains when the only sessions present are imports

Rewind already refuses imported checkpoints (refuseIfImportedCheckpoint).