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).

Cargo.lock

@@ -653,6 +653,15 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "crossbeam-channel"
+version = "0.5.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "82b8f8f868b36967f9606790d1903570de9ceaf870a7bf9fbbd3016d636a2cb2"
+dependencies = [
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "crossbeam-utils"
 version = "0.8.21"
@@ -3718,6 +3727,12 @@ version = "1.15.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "67b1b7a3b5fe4f1376887184045fcf45c69e92af734b7aaddc05fb777b6fbd03"
 
+[[package]]
+name = "smawk"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b7c388c1b5e93756d0c740965c41e8822f866621d41acbdf6336a6a168f8840c"
+
 [[package]]
 name = "socket2"
 version = "0.6.3"
@@ -3861,13 +3876,16 @@ dependencies = [
  "anyhow",
  "chrono",
  "clap",
+ "crossbeam-channel",
  "crossterm",
+ "futures",
  "ratatui",
  "rustls",
  "serde",
  "serde_json",
  "sunbeam-proto",
  "sunbeam-sdk",
+ "textwrap",
  "tokio",
  "tokio-stream",
  "toml",
@@ -3978,6 +3996,17 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "textwrap"
+version = "0.16.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c13547615a44dc9c452a8a534638acdf07120d4b6847c8178705da06306a3057"
+dependencies = [
+ "smawk",
+ "unicode-linebreak",
+ "unicode-width 0.2.0",
+]
+
 [[package]]
 name = "thiserror"
 version = "1.0.69"
@@ -4438,6 +4467,12 @@ version = "1.0.24"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e6e4313cd5fcd3dad5cafa179702e2b244f760991f45397d14d4ebf38247da75"
 
+[[package]]
+name = "unicode-linebreak"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3b09c83c3c29d37506a3e260c08c03743a6bb66a9cd432c6934ab501a190571f"
+
 [[package]]
 name = "unicode-segmentation"
 version = "1.12.0"