fix(tui): mark disposed when the agent leaves the registry under the TUI

agent/disposed only cleared the status line, so an agent-loop-only reload that
disposed the agent while the TUI stayed mounted left the local disposed flag
false. Since retained agents accept deliveries after detachment, later input
drove a zombie agent/session. Set disposed on agent/disposed so dispatchMessage
reports it. Adds a regression that sends after disposal and asserts no delivery.

packages/ui/tui/src/index.ts

@@ -3217,6 +3217,11 @@ export function createTuiChat(
   })
   const disposeAgent = ctx.on('agent/disposed', (subject) => {
     if (subject !== agent) return
+    // The agent left the registry (e.g. an agent-loop-only reload) while the
+    // TUI stays mounted. Retained agents accept deliveries after detachment, so
+    // without this a later send would drive a zombie agent/session; mark
+    // disposed so dispatchMessage reports it instead.
+    disposed = true
     clearStatus()
     appendNotice(`Agent "${agent.id}" was disposed.`, 'warning')
   })