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/adapters/agent.ts

@@ -19,8 +19,10 @@ import type {
   AgentGateway,
   CreateAgentInput,
   OpenTerminalOptions,
+  ReattachResult,
   TerminalHandle,
 } from "@/ports";
+import { makeTerminalHandle } from "./terminal";
 
 /** Wire shape returned by the `launch_agent` command (mirrors `open_terminal`). */
 interface LaunchAgentResponse {
@@ -87,22 +89,26 @@ export class TauriAgentGateway implements AgentGateway {
       onOutput: channel,
     });
 
-    const sessionId = res.sessionId;
+    return makeTerminalHandle(res.sessionId, channel);
+  }
+
+  async reattach(
+    sessionId: string,
+    onData: (bytes: Uint8Array) => void,
+  ): Promise<ReattachResult> {
+    // Agent sessions reattach through the same session-based `reattach_terminal`
+    // command as plain terminals (the PTY is identified by its session id).
+    const channel = new Channel<number[]>();
+    channel.onmessage = (chunk) => onData(Uint8Array.from(chunk));
+
+    const res = await invoke<{ sessionId: string; scrollback: number[] }>(
+      "reattach_terminal",
+      { sessionId, onOutput: channel },
+    );
+
     return {
-      sessionId,
-      async write(data: Uint8Array): Promise<void> {
-        await invoke("write_terminal", {
-          request: { sessionId, data: Array.from(data) },
-        });
-      },
-      async resize(rows: number, cols: number): Promise<void> {
-        await invoke("resize_terminal", {
-          request: { sessionId, rows, cols },
-        });
-      },
-      async close(): Promise<void> {
-        await invoke("close_terminal", { sessionId });
-      },
+      handle: makeTerminalHandle(res.sessionId, channel),
+      scrollback: Uint8Array.from(res.scrollback),
     };
   }
 }