feat(code): async agent bus, virtual viewport, event drain

- Agent service (crossbeam channels): TUI never blocks on gRPC I/O.
  Chat runs on a background tokio task, events flow back via bounded
  crossbeam channel. Designed as a library-friendly internal RPC.

- Virtual viewport: pre-wrap text with textwrap on content/width change,
  slice only visible rows for rendering. Paragraph gets no Wrap, no
  scroll() — pure O(viewport) per frame.

- Event drain loop: coalesce all queued terminal events before drawing.
  Filters MouseEventKind::Moved (crossterm's EnableMouseCapture floods
  these via ?1003h any-event tracking). Single redraw per batch.

- Conditional drawing: skip frames when nothing changed (needs_redraw).

- Mouse wheel + PageUp/Down + Home/End scrolling, command history
  (Up/Down, persistent to .sunbeam/history), Alt+L debug log overlay.

- Proto: SessionReady now includes history entries + resumed flag.
  Session resume loads conversation from Matrix room on reconnect.

- Default model: devstral-small-latest (was devstral-small-2506).

sunbeam-proto/proto/code.proto

@@ -66,6 +66,13 @@ message SessionReady {
   string session_id = 1;
   string room_id = 2;
   string model = 3;
+  bool resumed = 4;
+  repeated HistoryEntry history = 5;
+}
+
+message HistoryEntry {
+  string role = 1;    // "user" or "assistant"
+  string content = 2;
 }
 
 message TextDelta {