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/components/transcript-debug/index.ts

@@ -13,4 +13,5 @@ export { default as PermissionApproval } from './PermissionApproval.vue'
 export { default as PlanApproval } from './PlanApproval.vue'
 export { default as CodeBlock } from './CodeBlock.vue'
 export { default as AgentBadge } from './AgentBadge.vue'
+export { default as ResumeTerminalButton } from './ResumeTerminalButton.vue'
 export { AquaticBackground } from './aquaticBackground'