feat(onboarding): welcome eligible new players

design/log.md

@@ -49,3 +49,10 @@
 - Ambiguous, cancelled, zero-damage, environmental, and self damage do not opt out a player.
 - Combat opt-out bypasses administrative locks, immediately reconciles effect and prefix state, persists once, and sends one explanatory message.
 - Verified attribution, victim safety, lock bypass, duplicate suppression, and the full build with `./gradlew clean check jar`.
+
+### US-004 onboarding completed
+
+- Join observation now sends the configurable welcome message on each login before the original first-join instant plus the configured calendar-day window.
+- The exact expiration boundary and globally disabled behavior suppress reminders without resetting first-join state.
+- The default message names all player commands and explains automatic combat opt-out.
+- Verified repeated joins, expiration, global suppression, first-join retention, and the full build with `./gradlew clean check jar`.