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/domain/src/ports.rs

@@ -334,10 +334,24 @@ pub trait PtyPort: Send + Sync {
 
     /// Subscribes to the PTY's byte output stream.
     ///
+    /// Re-subscribable: each call returns a fresh stream that receives every
+    /// chunk produced **from now on**. Combined with [`scrollback`](Self::scrollback)
+    /// this lets the presentation layer *re-attach* a view to a still-living PTY
+    /// after a navigation/layout change tore the previous view down — without
+    /// re-spawning the process.
+    ///
     /// # Errors
     /// [`PtyError`] if the handle is unknown.
     fn subscribe_output(&self, handle: &PtyHandle) -> Result<OutputStream, PtyError>;
 
+    /// Returns the recent output retained for the session (a bounded scrollback
+    /// ring buffer, the most recent bytes). Used to repaint a view that
+    /// re-attaches to a live PTY so the terminal isn't blank.
+    ///
+    /// # Errors
+    /// [`PtyError`] if the handle is unknown.
+    fn scrollback(&self, handle: &PtyHandle) -> Result<Vec<u8>, PtyError>;
+
     /// Kills the PTY's process, returning its exit status.
     ///
     /// # Errors