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/application/src/terminal/registry.rs

@@ -59,6 +59,18 @@ impl TerminalSessions {
             .and_then(|m| m.get(id).map(|e| e.session.clone()))
     }
 
+    /// Returns the [`PtyHandle`]s of every currently-registered session.
+    ///
+    /// Used at application shutdown to kill all live PTYs cleanly (the
+    /// `CloseRequested` hook), independently of the frontend's per-view lifecycle.
+    #[must_use]
+    pub fn handles(&self) -> Vec<PtyHandle> {
+        self.entries
+            .lock()
+            .map(|m| m.values().map(|e| e.handle.clone()).collect())
+            .unwrap_or_default()
+    }
+
     /// Removes a session from the registry, returning its handle if present.
     pub fn remove(&self, id: &SessionId) -> Option<PtyHandle> {
         self.entries

crates/application/tests/agent_lifecycle.rs

@@ -313,6 +313,9 @@ impl PtyPort for FakePty {
     fn subscribe_output(&self, _handle: &PtyHandle) -> Result<OutputStream, PtyError> {
         Ok(Box::new(std::iter::empty()))
     }
+    fn scrollback(&self, _handle: &PtyHandle) -> Result<Vec<u8>, PtyError> {
+        Ok(Vec::new())
+    }
     async fn kill(&self, _handle: &PtyHandle) -> Result<ExitStatus, PtyError> {
         Ok(ExitStatus { code: Some(0) })
     }

crates/application/tests/terminal_usecases.rs

@@ -102,6 +102,10 @@ impl PtyPort for FakePty {
         Ok(Box::new(std::iter::empty()))
     }
 
+    fn scrollback(&self, _handle: &PtyHandle) -> Result<Vec<u8>, PtyError> {
+        Ok(Vec::new())
+    }
+
     async fn kill(&self, handle: &PtyHandle) -> Result<ExitStatus, PtyError> {
         let mut inner = self.0.lock().unwrap();
         inner.calls.push(Call::Kill {