fix(terminals): decouple PTY lifecycle from view lifecycle (no kill on navigation)

Navigating (layout/tab switch) tore the xterm view down and called
handle.close(), killing the backend PTY and cutting off running AIs. Now
the view's cleanup only detaches; only an explicit user action kills a PTY.

Backend:
- PortablePtyAdapter: per-session scrollback ring buffer (~100KB, most
  recent) + re-subscribable fan-out broadcast replacing the single-take
  output_rx. Reader thread feeds both the ring buffer and current
  subscribers; on EOF it closes subscribers (streams end) while keeping
  scrollback for late re-attach.
- PtyPort: new scrollback() method; subscribe_output is now re-subscribable
  (all impls + test fakes updated).
- reattach_terminal IPC command: returns scrollback and re-wires a fresh
  output channel on the live session without re-spawning.
- CloseRequested hook kills all live PTYs cleanly on app shutdown.
- TerminalSessions::handles() to enumerate live sessions at shutdown.

Frontend:
- TerminalHandle.detach(); TerminalGateway/AgentGateway.reattach() + mocks.
- TerminalView cleanup detaches (never close); on mount it re-attaches to a
  persisted session (repainting scrollback) instead of opening a new PTY.
- LayoutGrid persists the cell's session id via setSession; AgentsPanel
  tracks per-agent session ids — both drive reattach-vs-open.

Tests: ring buffer bounds to 100KB keeping newest bytes; scrollback retained;
re-subscription delivers post-reattach output; TerminalView detaches (not
closes) on unmount and reattaches with a known session; mock detach/reattach.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/app-tauri/tests/dto.rs

@@ -4,8 +4,8 @@
 
 use app_tauri_lib::dto::{
     parse_node_id, parse_session_id, ErrorDto, HealthRequestDto, HealthResponseDto, LayoutDto,
-    LayoutOperationDto, OpenTerminalRequestDto, ResizeTerminalRequestDto, TerminalClosedDto,
-    WriteTerminalRequestDto,
+    LayoutOperationDto, OpenTerminalRequestDto, ReattachResultDto, ResizeTerminalRequestDto,
+    TerminalClosedDto, WriteTerminalRequestDto,
 };
 use application::{CloseTerminalOutput, LayoutOperation, LoadLayoutOutput, OpenTerminalInput};
 use domain::{Direction, LayoutNode, LayoutTree, LeafCell, NodeId};
@@ -191,6 +191,16 @@ fn terminal_closed_dto_serialises_code_camel_case() {
     assert_eq!(serde_json::to_value(&none).unwrap(), json!({ "code": null }));
 }
 
+#[test]
+fn reattach_result_dto_serialises_camel_case() {
+    let dto = ReattachResultDto {
+        session_id: "sess-1".into(),
+        scrollback: vec![104, 105],
+    };
+    let v = serde_json::to_value(&dto).unwrap();
+    assert_eq!(v, json!({ "sessionId": "sess-1", "scrollback": [104, 105] }));
+}
+
 #[test]
 fn parse_session_id_accepts_uuid_and_rejects_garbage() {
     let sid = SessionId::from_uuid(Uuid::nil());