fix(engine): skip promoted-foreign commits in contamination audit

assertCleanBranchAtBase now checks each foreign-attributed commit
against `git merge-base --is-ancestor <sha> main`. If the commit is
already on local main, it was promoted through integration regardless
of whose Fusion-Task-Id trailer it carries — treating it as foreign
contamination is wrong and was the root cause of the FN-5475 cascade
(downstream worktrees inherited a sibling task's tip during the brief
fast-forward window before main moved further).

Audit cost: O(N) extra git calls per audit run, where N is the number
of foreign-trailer commits in baseSha..branchName. Each call is ~5-10ms
and N is typically 1-5. Negligible relative to the surrounding I/O.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/__tests__/branch-conflicts.test.ts

@@ -466,6 +466,48 @@ describe("branch-conflicts", () => {
     await expect(assertion).resolves.toBeUndefined();
   });
 
+  // FN-5475 / option-2 promotion check: a commit attributed to another
+  // task that's already reachable from local `main` was integrated via
+  // fast-forward and shouldn't be treated as contamination on a
+  // downstream branch that briefly inherited it.
+  it("assertCleanBranchAtBase treats foreign-attributed commits that are ancestors of main as promoted", async () => {
+    mockedExecSync.mockImplementation((cmd: string | string[]) => {
+      const command = typeof cmd === "string" ? cmd : cmd[0];
+      if (command.includes("git log --format=%H%x1f%s%x1f%b 'main..fusion/fn-4068'")) {
+        return Buffer.from("bbb222feat(FN-4386): foreignFusion-Task-Id: FN-4386\n");
+      }
+      if (command.includes("git merge-base --is-ancestor 'bbb222' 'main'")) {
+        // Simulate the FN-5475 case: foreign commit is already on main.
+        return Buffer.from("");
+      }
+      throw new Error(`Unexpected command: ${command}`);
+    });
+
+    await expect(
+      assertCleanBranchAtBase("/tmp/repo", "fusion/fn-4068", "main", "FN-4068"),
+    ).resolves.toBeUndefined();
+  });
+
+  it("assertCleanBranchAtBase still throws when foreign-attributed commits are NOT on main", async () => {
+    mockedExecSync.mockImplementation((cmd: string | string[]) => {
+      const command = typeof cmd === "string" ? cmd : cmd[0];
+      if (command.includes("git log --format=%H%x1f%s%x1f%b 'main..fusion/fn-4068'")) {
+        return Buffer.from("bbb222feat(FN-4386): foreignFusion-Task-Id: FN-4386\n");
+      }
+      if (command.includes("git merge-base --is-ancestor")) {
+        // Not on main — exits non-zero.
+        const err = new Error("not an ancestor") as Error & { stderr?: string };
+        err.stderr = "";
+        throw err;
+      }
+      throw new Error(`Unexpected command: ${command}`);
+    });
+
+    await expect(
+      assertCleanBranchAtBase("/tmp/repo", "fusion/fn-4068", "main", "FN-4068"),
+    ).rejects.toBeInstanceOf(BranchCrossContaminationError);
+  });
+
   describe("reportBranchAttribution", () => {
     const RS = "\x1e";
     const FS = "\x1f";

packages/engine/src/branch-conflicts.ts

@@ -400,6 +400,28 @@ export async function isBranchAuthoritativeForTask(
   return { ok: true };
 }
 
+/**
+ * Cheap ancestry check: is `commitSha` reachable from `ref`?
+ *
+ * Used to recognize "promoted" commits during contamination audits: when
+ * the engine fast-forwards local `main` with a sibling task's commit, that
+ * commit's `Fusion-Task-Id` trailer still points at the sibling, but the
+ * commit itself is now integrated. Treating it as foreign contamination
+ * for downstream tasks branched from the same main tip is incorrect — the
+ * commit is, by definition, ancestral on the integration target.
+ *
+ * Returns `false` on any git error (missing ref, repo unreadable, etc.)
+ * so the caller falls back to the conservative trailer-only judgement.
+ */
+async function isAncestorOf(repoDir: string, commitSha: string, ref: string): Promise<boolean> {
+  try {
+    await runGit(repoDir, `git merge-base --is-ancestor ${quoteShellArg(commitSha)} ${quoteShellArg(ref)}`);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
 export async function assertCleanBranchAtBase(
   repoDir: string,
   branchName: string,
@@ -412,17 +434,30 @@ export async function assertCleanBranchAtBase(
 
   const subjectPattern = /^(feat|fix|test|chore|docs|refactor|perf|build)\((FN-\d+)\):/i;
   const trailerPattern = /(?:^|\n)Fusion-Task-Id:\s*(FN-\d+)\s*(?:\n|$)/i;
-  const foreignCommits: BranchCrossContaminationCommit[] = [];
+  const candidateForeign: BranchCrossContaminationCommit[] = [];
   for (const line of output.split("\n").map((entry) => entry.trim()).filter(Boolean)) {
     const [sha, subject, body] = line.split("\u001f");
     const subjectMatch = (subject ?? "").match(subjectPattern);
     const trailerMatch = (body ?? "").match(trailerPattern);
     const attributedTaskId = (trailerMatch?.[1] ?? subjectMatch?.[2] ?? "").toUpperCase();
     if (attributedTaskId && attributedTaskId !== taskId.toUpperCase()) {
-      foreignCommits.push({ sha, subject: subject ?? "", foreignTaskId: attributedTaskId });
+      candidateForeign.push({ sha, subject: subject ?? "", foreignTaskId: attributedTaskId });
     }
   }
 
+  if (candidateForeign.length === 0) return;
+
+  // FN-5475: a commit attributed to another task that's already reachable
+  // from local `main` was promoted through integration. Treat it as
+  // ancestral, not contamination. This closes the race where a sibling
+  // task's commit briefly sat at local-main's tip while a downstream
+  // worktree was created (cross-task tip absorption).
+  const foreignCommits: BranchCrossContaminationCommit[] = [];
+  for (const commit of candidateForeign) {
+    if (await isAncestorOf(repoDir, commit.sha, "main")) continue;
+    foreignCommits.push(commit);
+  }
+
   if (foreignCommits.length > 0) {
     throw new BranchCrossContaminationError({ branchName, baseSha, taskId, foreignCommits });
   }