feat(tui): add agent interaction workflows

tui/src/prompt-queue.ts

@@ -0,0 +1,45 @@
+import type { MessageOptions } from "./protocol"
+
+export interface QueuedPrompt {
+  content: string
+  options: MessageOptions
+}
+
+/** Owns the difference between steering the active turn and starting the next one. */
+export class PromptQueue {
+  private prompts: QueuedPrompt[] = []
+  private armed = false
+
+  get length(): number {
+    return this.prompts.length
+  }
+
+  enqueue(prompt: QueuedPrompt): void {
+    this.prompts.push(prompt)
+    this.armed = true
+  }
+
+  /** A second Enter immediately promotes the newest queued prompt to steering. */
+  takeSteering(): QueuedPrompt | null {
+    if (!this.armed) return null
+    this.armed = false
+    return this.prompts.pop() ?? null
+  }
+
+  takeFollowUp(): QueuedPrompt | null {
+    this.armed = false
+    return this.prompts.shift() ?? null
+  }
+
+  restore(): QueuedPrompt[] {
+    const prompts = this.prompts
+    this.prompts = []
+    this.armed = false
+    return prompts
+  }
+
+  clear(): void {
+    this.prompts = []
+    this.armed = false
+  }
+}