FN-6102: guard spawnSync lookup in pi extensions

Avoid crashing when partial child_process mocks omit spawnSync.

- lazily load spawnSync from node:child_process via createRequire
- return null from Git linked worktree detection when spawnSync is unavailable
- add a regression test covering partial child_process mocks that only stub execSync

Files changed:
 packages/core/src/__tests__/pi-extensions.test.ts | 22 +++++++++++++++++++++-
 packages/core/src/pi-extensions.ts                | 18 +++++++++++++++++-
 2 files changed, 38 insertions(+), 2 deletions(-)

Fusion-Task-Id: FN-6102

Fusion-Task-Lineage: 22f98a1c-d296-4aa4-ac3d-7beead127dac

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

@@ -1,4 +1,4 @@
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 import { mkdtempSync, mkdirSync, realpathSync, rmSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
@@ -38,6 +38,26 @@ describe("getProjectRootFromWorktree", () => {
     ).toBe("/tmp");
   });
 
+  it("returns null without throwing when child_process partial mocks omit spawnSync", async () => {
+    vi.resetModules();
+    vi.doMock("node:child_process", () => ({
+      execSync: vi.fn(),
+    }));
+
+    try {
+      const { getProjectRootFromWorktree: getProjectRootFromWorktreeWithPartialMock } = await import(
+        "../pi-extensions.js"
+      );
+      const unmatchedPath = join(tmpdir(), "fn-6102-not-a-worktree");
+
+      expect(() => getProjectRootFromWorktreeWithPartialMock(unmatchedPath)).not.toThrow();
+      expect(getProjectRootFromWorktreeWithPartialMock(unmatchedPath)).toBe(null);
+    } finally {
+      vi.doUnmock("node:child_process");
+      vi.resetModules();
+    }
+  });
+
   it("detects arbitrary Git linked worktree paths when the parent has Fusion metadata", () => {
     const root = mkdtempSync(join(tmpdir(), "fn-6079-root-"));
     const worktreeRoot = mkdtempSync(join(tmpdir(), "fusion-ai-merge-fn-6079-"));

packages/core/src/pi-extensions.ts

@@ -1,9 +1,20 @@
 import { existsSync, mkdirSync, readdirSync, readFileSync, statSync, writeFileSync } from "node:fs";
 import { homedir } from "node:os";
 import { basename, dirname, isAbsolute, join, relative, resolve, sep, win32 } from "node:path";
-import { spawnSync } from "node:child_process";
+import { createRequire } from "node:module";
 
 const FUSION_DISABLED_EXTENSIONS_KEY = "fusionDisabledExtensions";
+const require = createRequire(import.meta.url);
+
+let cachedSpawnSync: typeof import("node:child_process")["spawnSync"] | undefined;
+let didLoadSpawnSync = false;
+function getSpawnSync(): typeof import("node:child_process")["spawnSync"] | undefined {
+  if (!didLoadSpawnSync) {
+    didLoadSpawnSync = true;
+    cachedSpawnSync = require("node:child_process").spawnSync;
+  }
+  return cachedSpawnSync;
+}
 
 export type PiExtensionSource = "fusion-global" | "pi-global" | "fusion-project" | "pi-project" | "package";
 
@@ -76,6 +87,11 @@ export function getProjectRootFromWorktree(
 }
 
 function getProjectRootFromGitLinkedWorktree(cwd: string): string | null {
+  const spawnSync = getSpawnSync();
+  if (!spawnSync) {
+    return null;
+  }
+
   const resolvedCwd = resolve(cwd);
   const commonDir = spawnSync("git", ["rev-parse", "--git-common-dir"], {
     cwd: resolvedCwd,