feat(FN-4606): complete Step 4 — enhance worktree root lookup

Ref: Runfusion/Fusion#321
Fusion-Task-Id: FN-4606
Fusion-Task-Lineage: 94b23f37-944e-4bcd-bfa1-67d0a8c0a267

packages/engine/src/__tests__/pi.test.ts

@@ -88,6 +88,20 @@ describe("getProjectRootFromWorktree", () => {
     expect(getProjectRootFromWorktree("C:\\repo\\.worktrees\\fn-001")).toBe("C:\\repo");
     expect(getProjectRootFromWorktree("C:\\repo\\.worktrees\\fn-001\\src\\file.ts")).toBe("C:\\repo");
   });
+
+  it("supports configured candidate worktrees dir paths", () => {
+    expect(
+      getProjectRootFromWorktree("/tmp/.fn-worktrees/repo/fn-001/src", {
+        worktreesDirCandidates: ["/tmp/.fn-worktrees/repo"],
+      }),
+    ).toBe("/tmp/.fn-worktrees");
+
+    expect(
+      getProjectRootFromWorktree("/tmp/repo.worktrees/fn-001", {
+        worktreesDirCandidates: ["/tmp/repo.worktrees"],
+      }),
+    ).toBe("/tmp");
+  });
 });
 
 // Initialize mocks before first test

packages/engine/src/pi.ts

@@ -1242,15 +1242,30 @@ async function registerExtensionProviders(cwd: string, modelRegistry: ModelRegis
  *   `/project/.worktrees/fn-001/src/file.ts` → `/project`
  *   `/project` → null (not a worktree)
  */
-export function getProjectRootFromWorktree(cwd: string): string | null {
-  // Match paths like:
-  //   /project/.worktrees/task-id
-  //   /project/.worktrees/task-id/src/file.ts
-  //   C:\project\.worktrees\task-id
-  const match = cwd.match(/^(.+?)[\\/]\.worktrees[\\/][^\\/]+(?:[\\/]|$)/);
-  if (match) {
-    return match[1]!;
+export function getProjectRootFromWorktree(
+  cwd: string,
+  opts?: { worktreesDirCandidates?: string[] },
+): string | null {
+  const legacyMatch = cwd.match(/^(.+?)[\\/]\.worktrees[\\/][^\\/]+(?:[\\/]|$)/);
+  if (legacyMatch) {
+    return legacyMatch[1]!;
   }
+
+  for (const candidate of opts?.worktreesDirCandidates ?? []) {
+    const normalizedCandidate = resolve(candidate);
+    const normalizedCwd = resolve(cwd);
+    const rel = relative(normalizedCandidate, normalizedCwd);
+    if (rel !== "" && !rel.startsWith("..") && !isAbsolute(rel)) {
+      const firstSegment = rel.split(/[\\/]/).filter(Boolean)[0];
+      if (firstSegment) {
+        const parent = normalizedCandidate.split(/[\\/]/).slice(0, -1).join("/");
+        if (parent) {
+          return parent;
+        }
+      }
+    }
+  }
+
   return null;
 }