feat(FN-5220): guard explicit duplicate markers in triage and self-healing

Adds an explicit duplicate-marker guard (FN-5220) spanning core helper, dashboard API endpoint, triage short-circuit, and self-healing sweep to detect and handle duplicate task creation attempts; includes comprehensive test coverage across unit, API, and integration layers plus documentation.

Fusion-Task-Id: FN-5220

packages/core/src/__tests__/explicit-duplicate-marker.test.ts

@@ -0,0 +1,70 @@
+import { describe, expect, it } from "vitest";
+
+import { parseExplicitDuplicateMarker } from "../explicit-duplicate-marker.js";
+
+const FULL_PROMPT = `# Task: FN-5211 - Example
+
+## Mission
+This is a duplicate-handling task, but it is a full spec body.
+
+- Implement the fix
+- Verify the result
+`;
+
+describe("parseExplicitDuplicateMarker", () => {
+  it("parses a canonical marker", () => {
+    expect(parseExplicitDuplicateMarker("DUPLICATE: FN-5211")).toEqual({
+      canonicalId: "FN-5211",
+    });
+  });
+
+  it("normalizes case and surrounding whitespace", () => {
+    expect(parseExplicitDuplicateMarker("   duplicate: fn-5211\n")).toEqual({
+      canonicalId: "FN-5211",
+    });
+  });
+
+  it("parses a backtick-wrapped marker", () => {
+    expect(parseExplicitDuplicateMarker("`DUPLICATE: FN-5211`")).toEqual({
+      canonicalId: "FN-5211",
+    });
+  });
+
+  it("parses a bold-wrapped marker", () => {
+    expect(parseExplicitDuplicateMarker("**DUPLICATE: FN-5211**")).toEqual({
+      canonicalId: "FN-5211",
+    });
+  });
+
+  it("parses a marker padded by blank lines", () => {
+    expect(parseExplicitDuplicateMarker("\n\n\nDUPLICATE: FN-5211\n\n")).toEqual({
+      canonicalId: "FN-5211",
+    });
+  });
+
+  it("parses a fenced marker", () => {
+    expect(parseExplicitDuplicateMarker("```text\nDUPLICATE: FN-5211\n```")).toEqual({
+      canonicalId: "FN-5211",
+    });
+  });
+
+  it("rejects a full prompt body that merely mentions duplicate", () => {
+    expect(parseExplicitDuplicateMarker(FULL_PROMPT)).toBeNull();
+  });
+
+  it("rejects extra prose after the marker", () => {
+    expect(parseExplicitDuplicateMarker("DUPLICATE: FN-5211\n\nSee also FN-5212")).toBeNull();
+  });
+
+  it("rejects multiple markers", () => {
+    expect(parseExplicitDuplicateMarker("DUPLICATE: FN-5211\nDUPLICATE: FN-5212")).toBeNull();
+  });
+
+  it("rejects empty input", () => {
+    expect(parseExplicitDuplicateMarker("   ")).toBeNull();
+  });
+
+  it("rejects non-FN identifiers", () => {
+    expect(parseExplicitDuplicateMarker("DUPLICATE: NOT-1234")).toBeNull();
+  });
+});

packages/core/src/explicit-duplicate-marker.ts

@@ -0,0 +1,52 @@
+export interface ExplicitDuplicateMarker {
+  canonicalId: string;
+}
+
+function stripCodeFenceLayer(content: string): string {
+  const fenceMatch = content.match(/^```(?:[\t ]*(?:text|markdown))?[\t ]*\n([\s\S]*?)\n```$/i);
+  if (!fenceMatch) {
+    return content;
+  }
+  return fenceMatch[1] ?? "";
+}
+
+function stripSingleWrapper(line: string): string {
+  if (line.startsWith("`") && line.endsWith("`") && line.length >= 2) {
+    return line.slice(1, -1).trim();
+  }
+  if (line.startsWith("**") && line.endsWith("**") && line.length >= 4) {
+    return line.slice(2, -2).trim();
+  }
+  return line;
+}
+
+/**
+ * Detects the canonical triage "redirect" marker emitted by the planning
+ * agent when the new task duplicates an existing one.
+ */
+export function parseExplicitDuplicateMarker(content: string): ExplicitDuplicateMarker | null {
+  const trimmed = content.trim();
+  if (!trimmed) {
+    return null;
+  }
+
+  const withoutFence = stripCodeFenceLayer(trimmed).trim();
+  const nonBlankLines = withoutFence
+    .split("\n")
+    .map((line) => line.trim())
+    .filter((line) => line.length > 0);
+
+  if (nonBlankLines.length !== 1) {
+    return null;
+  }
+
+  const candidate = stripSingleWrapper(nonBlankLines[0] ?? "");
+  const match = candidate.match(/^DUPLICATE:\s*(FN-\d+)\s*$/i);
+  if (!match) {
+    return null;
+  }
+
+  return {
+    canonicalId: match[1].toUpperCase(),
+  };
+}

packages/core/src/index.ts

@@ -156,6 +156,10 @@ export {
   type NearDuplicateMatch,
 } from "./near-duplicate.js";
 export { getTaskDuplicateLineage } from "./duplicate-lineage.js";
+export {
+  parseExplicitDuplicateMarker,
+  type ExplicitDuplicateMarker,
+} from "./explicit-duplicate-marker.js";
 export {
   __getDeterministicGuardMutexSize,
   deterministicGuardLocks,