feat: ResumeTerminalButton with ephemeral terminal for transcript-debug

- Add ResumeTerminalButton component with floating terminal modal
  (drag, resize, glass morphism, TerminalNavButtons bar)
- Add useEphemeralTerminal composable for temporary PTY sessions
  that auto-run `<agent> --resume <sessionId>` and cleanup on close
- Add /kill-session POST endpoint to terminal server for ephemeral
  session cleanup
- Integrate button in ChatContainer header (ID row) and status bar
- Pass selectedAgent to ChatContainer from TranscriptDebugPage

frontend/src/pages/TranscriptDebugPage.vue

@@ -109,6 +109,7 @@ onUnmounted(() => {
             v-if="conversation"
             :conversation="conversation"
             :processing="processing"
+            :selected-agent="selectedAgent"
             @send="handleSend"
           />
         </div>