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>

frontend/src/features/agents/AgentsPanel.tsx

@@ -73,6 +73,15 @@ export function AgentsPanel({ projectId, projectRoot = "" }: AgentsPanelProps) {
    */
   const [activeAgentId, setActiveAgentId] = useState<string | null>(null);
 
+  /**
+   * Live PTY session id of the running agent terminal, keyed by agent id. Lets
+   * the terminal re-attach (rather than re-launch) when the panel re-mounts the
+   * view, so navigating away never kills the agent.
+   */
+  const [agentSessions, setAgentSessions] = useState<Record<string, string>>(
+    {},
+  );
+
   const canCreate = newName.trim().length > 0 && !vm.busy;
 
   async function handleCreate(e: React.FormEvent) {
@@ -325,10 +334,18 @@ export function AgentsPanel({ projectId, projectRoot = "" }: AgentsPanelProps) {
         <div className="border-t border-border p-4">
           <div className="h-96 overflow-hidden rounded-lg border border-border bg-surface">
             <TerminalView
+              key={activeAgentId}
               cwd={projectRoot}
               open={(opts, onData) =>
                 vm.launchAgent(activeAgentId, opts, onData)
               }
+              reattach={(sessionId, onData) =>
+                gateways.agent.reattach(sessionId, onData)
+              }
+              sessionId={agentSessions[activeAgentId] ?? null}
+              onSessionId={(sid) =>
+                setAgentSessions((prev) => ({ ...prev, [activeAgentId]: sid }))
+              }
             />
           </div>
         </div>