test(FN-4904): add core resolver integration coverage

Fusion-Task-Id: FN-4904
Fusion-Task-Lineage: 5da52c8c-0ed6-4e3a-920e-aa1b56d0b719

packages/core/src/__tests__/pi-extensions.test.ts

@@ -1,5 +1,8 @@
 import { describe, expect, it } from "vitest";
-import { getProjectRootFromWorktree } from "../pi-extensions.js";
+import { mkdtempSync, mkdirSync, rmSync } from "node:fs";
+import { join } from "node:path";
+import { tmpdir } from "node:os";
+import { getProjectRootFromWorktree, resolvePiExtensionProjectRoot } from "../pi-extensions.js";
 
 describe("getProjectRootFromWorktree", () => {
   it("detects POSIX worktree paths", () => {
@@ -26,3 +29,46 @@ describe("getProjectRootFromWorktree", () => {
     ).toBe("/tmp");
   });
 });
+
+describe("resolvePiExtensionProjectRoot", () => {
+  it("prefers parent repo root for worktree paths when parent .fusion exists", () => {
+    const root = mkdtempSync(join(tmpdir(), "fn-4904-root-"));
+    try {
+      mkdirSync(join(root, ".fusion"), { recursive: true });
+      mkdirSync(join(root, ".worktrees", "feature", ".fusion"), { recursive: true });
+      const cwd = join(root, ".worktrees", "feature", "sub");
+      mkdirSync(cwd, { recursive: true });
+
+      expect(resolvePiExtensionProjectRoot(cwd)).toBe(root);
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+
+  it("falls back to legacy walk when parent repo root does not have .fusion", () => {
+    const root = mkdtempSync(join(tmpdir(), "fn-4904-root-"));
+    try {
+      const worktreeRoot = join(root, ".worktrees", "feature");
+      mkdirSync(join(worktreeRoot, ".fusion"), { recursive: true });
+      const cwd = join(worktreeRoot, "sub");
+      mkdirSync(cwd, { recursive: true });
+
+      expect(resolvePiExtensionProjectRoot(cwd)).toBe(worktreeRoot);
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+
+  it("preserves non-worktree behavior", () => {
+    const root = mkdtempSync(join(tmpdir(), "fn-4904-root-"));
+    try {
+      mkdirSync(join(root, ".fusion"), { recursive: true });
+      const cwd = join(root, "sub", "dir");
+      mkdirSync(cwd, { recursive: true });
+
+      expect(resolvePiExtensionProjectRoot(cwd)).toBe(root);
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+});