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.

bin/mempalace-pi-session

@@ -212,7 +212,11 @@ Options:
                          assistant text, tool results excluded (default: 1000).
                          Catches abandoned sessions whose bulk is injected
                          skill/context text in the user prompt.
-  --agent <name>         Agent name recorded on drawers (default: $USER)
+  --agent <name>         Agent name recorded on drawers. Defaults to
+                         pi@$MEMPALACE_PI_DEVICE when that is set, else $USER.
+                         The palace records neither the harness nor the machine
+                         on a write, so this one string is what makes a drawer
+                         attributable to both.
   --sessions-dir <path>  Path to pi sessions dir (default: $PI_SESSIONS_DIR
                          or ~/.pi/agent/sessions)
   --stage <path>         Staging root (default: $MEMPALACE_PI_STAGE, else