fix(HAI-039): eliminate duplicate description in prompts and modal heading

- Fix prompt heading to use only task ID when title is absent, preventing description duplication
- Strip leading markdown heading from TaskDetailModal since the modal has its own header
- Add store tests covering prompt generation for tasks with and without titles
- Update TaskDetailModal tests to verify heading stripping and single description rendering

packages/core/src/store.test.ts

@@ -55,6 +55,58 @@ describe("TaskStore", () => {
     return task;
   }
 
+  // ── Prompt generation (no duplicate description) ───────────────
+
+  describe("prompt generation", () => {
+    it("triage task without title does not duplicate description in PROMPT.md", async () => {
+      const task = await store.createTask({ description: "Fix the login bug" });
+      const detail = await store.getTask(task.id);
+
+      // Heading should be just the ID, not the description
+      expect(detail.prompt).toMatch(/^# HAI-001\n/);
+      // Description appears exactly once
+      const count = detail.prompt.split("Fix the login bug").length - 1;
+      expect(count).toBe(1);
+    });
+
+    it("triage task with title uses title in heading and description in body", async () => {
+      const task = await store.createTask({
+        title: "Login bug",
+        description: "Fix the login bug on the settings page",
+      });
+      const detail = await store.getTask(task.id);
+
+      expect(detail.prompt).toMatch(/^# HAI-001: Login bug\n/);
+      expect(detail.prompt).toContain("Fix the login bug on the settings page");
+    });
+
+    it("generateSpecifiedPrompt does not duplicate when title is absent", async () => {
+      const task = await store.createTask({
+        description: "Implement caching layer",
+        column: "todo",
+      });
+      const detail = await store.getTask(task.id);
+
+      // Heading should be just the ID
+      expect(detail.prompt).toMatch(/^# HAI-001\n/);
+      // Description appears exactly once (in Mission section)
+      const count = detail.prompt.split("Implement caching layer").length - 1;
+      expect(count).toBe(1);
+    });
+
+    it("generateSpecifiedPrompt uses title in heading when present", async () => {
+      const task = await store.createTask({
+        title: "Add caching",
+        description: "Implement caching layer for API responses",
+        column: "todo",
+      });
+      const detail = await store.getTask(task.id);
+
+      expect(detail.prompt).toMatch(/^# HAI-001: Add caching\n/);
+      expect(detail.prompt).toContain("Implement caching layer for API responses");
+    });
+  });
+
   // ── Lock serialization test ──────────────────────────────────────
 
   describe("write lock serialization", () => {

packages/core/src/store.ts

@@ -197,9 +197,9 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     // Update cache if watcher is active
     if (this.watcher) this.taskCache.set(id, { ...task });
 
-    const heading = task.title || task.description;
+    const heading = task.title ? `${id}: ${task.title}` : id;
     const prompt = task.column === "triage"
-      ? `# ${id}: ${heading}\n\n${task.description}\n`
+      ? `# ${heading}\n\n${task.description}\n`
       : this.generateSpecifiedPrompt(task);
     await writeFile(join(dir, "PROMPT.md"), prompt);
 
@@ -835,15 +835,15 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
         ? task.dependencies.map((d) => `- **Task:** ${d}`).join("\n")
         : "- **None**";
 
-    const heading = task.title || task.description;
+    const heading = task.title ? `${task.id}: ${task.title}` : task.id;
-    return `# ${task.id}: ${heading}
+    return `# ${heading}
 
 **Created:** ${task.createdAt.split("T")[0]}
 **Size:** M
 
 ## Mission
 
-${task.description || task.title}
+${task.description}
 
 ## Dependencies
 

packages/dashboard/app/components/TaskDetailModal.tsx

@@ -154,7 +154,7 @@ export function TaskDetailModal({
             {task.prompt ? (
               <div className="markdown-body">
                 <ReactMarkdown remarkPlugins={[remarkGfm]}>
-                  {task.prompt}
+                  {task.prompt.replace(/^#\s+[^\n]*\n+/, "")}
                 </ReactMarkdown>
               </div>
             ) : (

packages/dashboard/app/components/__tests__/TaskDetailModal.test.tsx

@@ -41,7 +41,7 @@ describe("TaskDetailModal", () => {
     expect(markdownDiv!.classList.contains("detail-prompt")).toBe(false);
   });
 
-  it("renders ReactMarkdown output (heading and bold text)", () => {
+  it("strips the leading heading from prompt and renders remaining markdown", () => {
     const { container } = render(
       <TaskDetailModal
         task={makeTask({ prompt: "# Hello\n\nSome **bold** text" })}
@@ -53,7 +53,8 @@ describe("TaskDetailModal", () => {
       />,
     );
 
-    expect(container.querySelector("h1")?.textContent).toBe("Hello");
+    // The leading # heading should be stripped (modal has its own header)
+    expect(container.querySelector(".markdown-body h1")).toBeNull();
     expect(container.querySelector("strong")?.textContent).toBe("bold");
   });
 
@@ -89,4 +90,29 @@ describe("TaskDetailModal", () => {
 
     expect(screen.queryByText("PROMPT.md")).toBeNull();
   });
+
+  it("shows description exactly once for a task without title", () => {
+    const { container } = render(
+      <TaskDetailModal
+        task={makeTask({
+          title: undefined,
+          description: "Fix the login bug",
+          prompt: "# HAI-099\n\nFix the login bug\n",
+        })}
+        onClose={noop}
+        onMoveTask={noopMove}
+        onDeleteTask={noopDelete}
+        onMergeTask={noopMerge}
+        addToast={noop}
+      />,
+    );
+
+    // The heading "HAI-099" should be stripped from the markdown
+    const markdownBody = container.querySelector(".markdown-body");
+    expect(markdownBody?.innerHTML).not.toContain("HAI-099");
+    // Description appears in the markdown body
+    expect(markdownBody?.textContent).toContain("Fix the login bug");
+    // The detail header shows the ID (not duplicated as markdown heading)
+    expect(container.querySelector(".detail-id")?.textContent).toBe("HAI-099");
+  });
 });