Long-press message selection + delete (message.delete frame, outbox removal, all-device sync)

docs/10-android-app.md

@@ -126,6 +126,20 @@ app/shared/src/
 ### Intermediate messages
 - `commentary` frames → dimmed/smaller bubble, distinct from final answers.
 
+### Message selection + delete
+- **Long-press** a message bubble (touch) or **right-click** it (desktop mouse)
+  enters selection mode: the tapped message is selected (a circular check appears
+  beside each bubble) and the composer is replaced by a selection toolbar
+  (count + **Delete** + cancel ✕).
+- While selecting, **tap** a bubble to toggle it; the ✕ (or deselecting the last
+  message) exits selection mode. Only finalized messages are selectable — a
+  streaming bubble has no final id yet and a pending echo isn't on the server.
+- **Delete** → confirm dialog → `message.delete {message_ids:[…]}` for the
+  current lane. The server removes the message(s) from the outbox (so
+  `history`/`sync` no longer return them) and broadcasts `message.deleted` to
+  every device; each device drops them from its cache (the requesting device
+  also drops them locally for snappy UX). Deleting is idempotent.
+
 ### Agent busy / stop / steer
 - `agent.busy` → show "thinking…" indicator in chat header (animated dots).
 - `agent.idle` → clear indicator.