The migration moved ~/.pi-web/archived-sessions* into a custom
PI_WEB_DATA_DIR at session daemon startup. Remove it outright: data
directories are independent, and anyone who sets a custom data
directory can copy archived-sessions.json and archived-sessions/
manually while the daemon is stopped.
Without the migration, runSessionDaemonStartup's only remaining
contract was sequencing create/register/listen, so inline those steps
into sessiond.ts and drop the wrapper module and its tests.