feat: interactive ephemeral terminal per transcript session

Replace one-shot HTTP POST sendPrompt with a persistent ephemeral
terminal per session. Terminal auto-starts on session select, stays
running in background when modal is closed, and gets killed on
session switch or page unload.

- Add sendInput() to useEphemeralTerminal (text + Enter as separate WS messages)
- useTranscriptDebug owns terminal lifecycle (create/dispose on select/switch)
- ResumeTerminalButton receives shared terminal prop, only toggles modal
- UserInput shows "Starting terminal..." when not ready
- Add "New Session" button that starts a fresh agent session
- beforeunload sends sendBeacon to kill terminal on page close

frontend/src/pages/TranscriptDebugPage.vue

@@ -17,9 +17,12 @@ const {
   isRealtime,
   sending,
   processing,
+  ephemeral,
+  terminalReady,
   init,
   switchAgent,
   selectSession,
+  createNewSession,
   disconnectRealtime,
   sendPrompt
 } = useTranscriptDebug()
@@ -34,6 +37,10 @@ function handleSend(message: string) {
   sendPrompt(message)
 }
 
+function handleCreateSession() {
+  createNewSession()
+}
+
 onMounted(() => {
   init()
 })
@@ -109,8 +116,11 @@ onUnmounted(() => {
             v-if="conversation"
             :conversation="conversation"
             :processing="processing"
+            :terminal-ready="terminalReady"
+            :terminal="ephemeral"
             :selected-agent="selectedAgent"
             @send="handleSend"
+            @create-session="handleCreateSession"
           />
         </div>
       </div>