feat(FN-4884): complete Step 2-3 fast-path and coverage

Fusion-Task-Id: FN-4884
Fusion-Task-Lineage: b906ade4-a9f1-4f2e-b684-4ebf49c3e7e6

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

@@ -207,6 +207,47 @@ describe("branch contamination recovery classification", () => {
     expect(range).toBe("0");
   });
 
+  it("reanchors without checkout -B when detached worktree is already at base on bound branch", async () => {
+    const { repoDir, baseSha } = await setupRepo();
+    const secondaryWorktree = path.join(repoDir, "../feature-secondary");
+    await run(`git worktree add --detach ${JSON.stringify(secondaryWorktree)} ${baseSha}`, repoDir);
+    dirs.push(secondaryWorktree);
+
+    await expect(
+      run(`git checkout -B feature ${baseSha}`, secondaryWorktree),
+    ).rejects.toThrow(/already used by worktree/i);
+
+    const result = await reanchorBranchToBase({
+      repoDir,
+      worktreePath: secondaryWorktree,
+      branchName: "feature",
+      baseSha,
+      taskId: "FN-4884",
+    });
+
+    const headSha = await run("git rev-parse HEAD", secondaryWorktree);
+    expect(result.previousTipSha).toBe(baseSha);
+    expect(result.newTipSha).toBe(baseSha);
+    expect(headSha).toBe(baseSha);
+  });
+
+  it("treats already-bound branch-at-base as a no-op fast-path", async () => {
+    const { repoDir, baseSha } = await setupRepo();
+
+    const result = await reanchorBranchToBase({
+      repoDir,
+      worktreePath: repoDir,
+      branchName: "feature",
+      baseSha,
+      taskId: "FN-4884",
+    });
+
+    const currentBranch = await run("git symbolic-ref --quiet --short HEAD", repoDir);
+    expect(result.previousTipSha).toBe(baseSha);
+    expect(result.newTipSha).toBe(baseSha);
+    expect(currentBranch).toBe("feature");
+  });
+
   it("auto-recovers by dropping already-upstream foreign commits and preserving remaining branch work", async () => {
     const { repoDir, baseSha } = await setupRepo();
     await writeFile(path.join(repoDir, "foreign.txt"), "", "utf-8");

packages/engine/src/branch-conflicts.ts

@@ -558,6 +558,29 @@ export async function reanchorBranchToBase(
     timeout: GIT_TIMEOUT_MS,
     maxBuffer: GIT_MAX_BUFFER,
   });
+
+  const worktreeHeadSha = await revParse(worktreePath, "HEAD");
+  const branchTipSha = previousTipSha;
+  const worktreeHeadBranch = await runGit(worktreePath, "git symbolic-ref --quiet --short HEAD").catch(() => "");
+
+  if (worktreeHeadSha === baseSha && branchTipSha === baseSha) {
+    if (worktreeHeadBranch !== branchName) {
+      try {
+        await runGit(worktreePath, `git checkout ${quoteShellArg(branchName)}`);
+      } catch (error) {
+        const message = error instanceof Error ? error.message : String(error);
+        if (!message.includes("already used by worktree")) {
+          throw error;
+        }
+      }
+    }
+    await assertCleanBranchAtBase(repoDir, branchName, baseSha, taskId);
+    return {
+      previousTipSha,
+      newTipSha: previousTipSha,
+    };
+  }
+
   await runGit(worktreePath, `git checkout --detach ${quoteShellArg(baseSha)}`);
   await runGit(worktreePath, `git checkout -B ${quoteShellArg(branchName)} ${quoteShellArg(baseSha)}`);
   await assertCleanBranchAtBase(repoDir, branchName, baseSha, taskId);