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/Cargo.toml

@@ -25,3 +25,9 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 toml = "0.8"
 anyhow = "1"
+futures = "0.3"
+crossbeam-channel = "0.5"
+textwrap = "0.16"
+
+[dev-dependencies]
+tokio-stream = { version = "0.1", features = ["net"] }