FN-6116: clear terminal event buffers after subscriber delivery

Prevent stale terminal connection metadata from replaying after it has already been delivered.

- only retain scrollback and connected payloads when no listeners are registered yet
- clear buffered terminal events after live subscribers receive them
- add regression coverage for scrollback and connected replay behavior

Files changed:
 .../app/hooks/__tests__/useTerminal.test.ts        | 50 ++++++++++++++++++++++
 packages/dashboard/app/hooks/useTerminal.ts        | 23 ++++++----
 2 files changed, 65 insertions(+), 8 deletions(-)

Fusion-Task-Id: FN-6116

Fusion-Task-Lineage: 5876fe24-2e90-4ef1-9e8b-5abce40555b4

packages/dashboard/app/hooks/__tests__/useTerminal.test.ts

@@ -402,6 +402,56 @@ describe("useTerminal", () => {
       expect(onData).toHaveBeenCalledTimes(1);
     });
 
+    it("clears scrollback buffer after direct delivery to subscribers", () => {
+      const { result } = renderHook(() => useTerminal("test-session-123"));
+      const firstSubscription = vi.fn();
+      const secondSubscription = vi.fn();
+
+      let unsubscribeFirst: (() => void) | undefined;
+      act(() => {
+        unsubscribeFirst = result.current.onScrollback(firstSubscription);
+      });
+
+      act(() => {
+        MockWebSocket.instances[0].emitMessage({ type: "scrollback", data: "initial prompt$ " });
+      });
+
+      expect(firstSubscription).toHaveBeenCalledTimes(1);
+      expect(firstSubscription).toHaveBeenCalledWith("initial prompt$ ");
+
+      act(() => {
+        unsubscribeFirst?.();
+        result.current.onScrollback(secondSubscription);
+      });
+
+      expect(secondSubscription).not.toHaveBeenCalled();
+    });
+
+    it("clears connected buffer after direct delivery to subscribers", () => {
+      const { result } = renderHook(() => useTerminal("test-session-123"));
+      const firstSubscription = vi.fn();
+      const secondSubscription = vi.fn();
+
+      let unsubscribeFirst: (() => void) | undefined;
+      act(() => {
+        unsubscribeFirst = result.current.onConnect(firstSubscription);
+      });
+
+      act(() => {
+        MockWebSocket.instances[0].emitMessage({ type: "connected", shell: "/bin/bash", cwd: "/project" });
+      });
+
+      expect(firstSubscription).toHaveBeenCalledTimes(1);
+      expect(firstSubscription).toHaveBeenCalledWith({ shell: "/bin/bash", cwd: "/project" });
+
+      act(() => {
+        unsubscribeFirst?.();
+        result.current.onConnect(secondSubscription);
+      });
+
+      expect(secondSubscription).not.toHaveBeenCalled();
+    });
+
     it("clears buffered scrollback after first replay to prevent duplicate delivery", () => {
       const { result } = renderHook(() => useTerminal("test-session-123"));
 

packages/dashboard/app/hooks/useTerminal.ts

@@ -353,18 +353,25 @@ export function useTerminal(sessionId: string | null, projectId?: string): UseTe
             break;
           case "scrollback":
             if (msg.data) {
-              // Buffer scrollback for late subscribers
-              buffer.scrollback = msg.data;
-              onScrollbackCallbacksRef.current.forEach((cb) => cb(msg.data!));
+              // Buffer scrollback only when no subscribers are registered yet
+              if (onScrollbackCallbacksRef.current.size === 0) {
+                buffer.scrollback = msg.data;
+              } else {
+                onScrollbackCallbacksRef.current.forEach((cb) => cb(msg.data!));
+                buffer.scrollback = null;
+              }
             }
             break;
           case "connected":
             if (msg.shell && msg.cwd) {
-              // Buffer connected info for late subscribers
-              buffer.connected = { shell: msg.shell!, cwd: msg.cwd! };
-              onConnectCallbacksRef.current.forEach((cb) => 
-                cb({ shell: msg.shell!, cwd: msg.cwd! })
-              );
+              const connectedInfo = { shell: msg.shell!, cwd: msg.cwd! };
+              // Buffer connected info only when no subscribers are registered yet
+              if (onConnectCallbacksRef.current.size === 0) {
+                buffer.connected = connectedInfo;
+              } else {
+                onConnectCallbacksRef.current.forEach((cb) => cb(connectedInfo));
+                buffer.connected = null;
+              }
             }
             break;
           case "exit":