docs: MEMPALACE_PI_DEVICE now also attributes drawers

Follow-up to c64ffa1, which changed the --agent default but left --help
claiming $USER. Fixes that text and states in README/ARCHITECTURE that the
device label reaches the palace as added_by, since mempalace stores neither the
machine nor the harness on a write.

ARCHITECTURE.md

@@ -196,6 +196,14 @@ server to mine its own local copy of that inbox over the same HTTP
 `$MEMPALACE_PI_SSH_TARGET`; see `--help` for the rest
 (`MEMPALACE_PI_SSH_CONFIG`, `MEMPALACE_PI_REMOTE_PATH`, `MEMPALACE_PI_DEVICE`).
 
+**Attribution.** `MEMPALACE_PI_DEVICE` is not only the inbox directory name: it
+also defaults `--agent` to `pi@<device>`, which reaches the palace as each
+drawer's `added_by`. This matters because mempalace records neither the
+originating machine nor the harness on a write, and a shared bearer token leaves
+the server unable to tell clients apart. Without it, attribution has to be
+reconstructed after the fact from the inbox path and the `pi_*.jsonl` filename —
+which works for mined transcripts but not for anything filed by hand.
+
 **Filters:** two gates, both required — stricter than `mempalace-session`'s
 single filter because pi's transcripts have a failure mode opencode's don't: