feat(FN-5205): add workflow-step test mode dispatch and routing for mock pr

Implements workflow step test mode (FN-5205) by wiring mock dispatch, context forwarding, and routing through executor, merger, and mock provider, plus adding corresponding tests and docs. Also adds broad-scope triage heuristics to improve task-scope detection, touching triage.ts, triage-broad-scope

Fusion-Task-Id: FN-5205

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>
Fusion-Task-Id: FN-5205

scripts/__tests__/replace-seeded-workflow-prompts.test.mjs

@@ -1,6 +1,6 @@
 import test from "node:test";
 import assert from "node:assert/strict";
-import { mkdtempSync, rmSync } from "node:fs";
+import { mkdtempSync, rmSync, readFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join, resolve } from "node:path";
 import { execFileSync, spawnSync } from "node:child_process";
@@ -81,6 +81,12 @@ test("replaces seeded workflow prompts and remains idempotent", async () => {
   }
 });
 
+test("pins FN-5205 test-mode rationale comment", () => {
+  const source = readFileSync(scriptPath, "utf8");
+  assert.match(source, /FN-5205/);
+  assert.match(source, /Do not branch seeded prompt bodies\s*\n\/\/ on testMode\./);
+});
+
 test("reports skipped when seeded rows are absent", () => {
   ensureCoreDist();
   const tempDir = mkdtempSync(join(tmpdir(), "fn4493-script-empty-"));

scripts/replace-seeded-workflow-prompts.mjs

@@ -1,4 +1,10 @@
 #!/usr/bin/env node
+// Test-mode note (FN-5205):
+// Seeded prompt bodies (including WS-004 / browser-verification) are NOT
+// altered for test mode. Test-mode determinism is delivered at the provider
+// layer via the FN-5203 mock-provider script registry; the seeded prompt
+// remains the production canonical text. Do not branch seeded prompt bodies
+// on testMode.
 /**
  * Rollback prompts captured before replacement (from local worktree DB):
  * WS-004: not present in .fusion/fusion.db at capture time.