feat(FN-4717): complete Step 2 — add backend resolver precedence

Fusion-Task-Id: FN-4717
Fusion-Task-Lineage: a2533cd8-52c4-43a0-9daf-e0a49745306b

packages/engine/src/__tests__/worktree-backend.test.ts

@@ -608,6 +608,57 @@ describe("resolveWorktreeBackend", () => {
     expect(resolveWorktreeBackend({ worktrunk: { enabled: true, binaryPath: "worktrunk" } as any }).kind).toBe("worktrunk");
   });
 
+  it("uses literal binaryPath over resolver when both are provided", async () => {
+    execMock.mockResolvedValue({ stdout: "", stderr: "" });
+    const resolver = vi.fn().mockResolvedValue("/resolved");
+    const backend = resolveWorktreeBackend(
+      { worktrunk: { enabled: true, binaryPath: " /literal " } as any },
+      { binaryPathResolver: resolver },
+    );
+
+    await (backend as WorktrunkWorktreeBackend).remove({
+      rootDir: "/repo",
+      worktreePath: "/repo/.worktrees/fn-1",
+      branch: "fusion/fn-1",
+    });
+
+    expect(resolver).not.toHaveBeenCalled();
+    expect(execMock).toHaveBeenCalledWith(
+      '"/literal" "remove" "--foreground" "fusion/fn-1"',
+      expect.objectContaining({ cwd: "/repo" }),
+    );
+  });
+
+  it("wires binaryPathResolver when literal is absent", async () => {
+    execMock.mockResolvedValue({ stdout: "", stderr: "" });
+    const resolver = vi.fn().mockResolvedValue("/resolved");
+    const backend = resolveWorktreeBackend({ worktrunk: { enabled: true } as any }, { binaryPathResolver: resolver });
+
+    await (backend as WorktrunkWorktreeBackend).remove({
+      rootDir: "/repo",
+      worktreePath: "/repo/.worktrees/fn-1",
+      branch: "fusion/fn-1",
+    });
+
+    expect(resolver).toHaveBeenCalledTimes(1);
+    expect(execMock).toHaveBeenCalledWith(
+      '"/resolved" "remove" "--foreground" "fusion/fn-1"',
+      expect.objectContaining({ cwd: "/repo" }),
+    );
+  });
+
+  it("preserves null behavior when literal and resolver are absent", async () => {
+    const backend = resolveWorktreeBackend({ worktrunk: { enabled: true } as any });
+
+    await expect(
+      (backend as WorktrunkWorktreeBackend).remove({
+        rootDir: "/repo",
+        worktreePath: "/repo/.worktrees/fn-1",
+        branch: "fusion/fn-1",
+      }),
+    ).rejects.toMatchObject({ code: "worktrunk_binary_missing", operation: "remove" });
+  });
+
   it("uses worktrunk when enabled without binaryPath", () => {
     expect(resolveWorktreeBackend({ worktrunk: { enabled: true } as any }).kind).toBe("worktrunk");
   });

packages/engine/src/worktree-backend.ts

@@ -585,11 +585,17 @@ export async function removeWorktree(input: {
 
 export function resolveWorktreeBackend(
   settings: Partial<Settings>,
-  deps: { logger?: { log: (m: string) => void; warn: (m: string) => void } } = {},
+  deps: {
+    logger?: { log: (m: string) => void; warn: (m: string) => void };
+    binaryPathResolver?: () => Promise<string | null>;
+  } = {},
 ): WorktreeBackend {
   if (settings.worktrunk?.enabled === true) {
+    // FN-4681 wires binaryPathResolver from worktree-acquisition; precedence is literal setting > resolver > null.
+    const configuredBinaryPath = settings.worktrunk.binaryPath?.trim() ?? "";
+    const binaryPath = configuredBinaryPath ? configuredBinaryPath : deps.binaryPathResolver ?? null;
     return new WorktrunkWorktreeBackend({
-      binaryPath: settings.worktrunk.binaryPath ?? null,
+      binaryPath,
       logger: deps.logger,
     });
   }