fix: arr top now creates empty @ above stack for new work

Previously arr top just navigated to the topmost described change.
Now it creates an empty @ above the stack (if not already there),
matching the STORIES.md expectation: "Empty @ above stack top".

Uses jj directly to check if @ is empty/undescribed with no children.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jonathanlab

packages/core/src/jj.ts

@@ -804,30 +804,83 @@ export class JJ {
   }
 
   async navigateTop(): Promise<Result<NavigationResult>> {
-    // Use heads(descendants(@)) to find the tip of the current stack
+    // Check if @ is empty, undescribed, and has no children (already at top ready for work)
+    const wcResult = await this.run([
+      "log",
+      "-r",
+      "@",
+      "--no-graph",
+      "-T",
+      'empty ++ "\\t" ++ description.first_line()',
+    ]);
+    const childrenResult = await this.run([
+      "log",
+      "-r",
+      "@+",
+      "--no-graph",
+      "-T",
+      "change_id.short()",
+    ]);
+
+    if (wcResult.ok && childrenResult.ok) {
+      const [empty, desc = ""] = wcResult.value.stdout.trim().split("\t");
+      const hasChildren = childrenResult.value.stdout.trim() !== "";
+
+      if (empty === "true" && desc === "" && !hasChildren) {
+        // Already at top with empty @, return parent info
+        const parentResult = await this.run([
+          "log",
+          "-r",
+          "@-",
+          "--no-graph",
+          "-T",
+          'change_id.short() ++ "\\t" ++ description.first_line()',
+        ]);
+        if (parentResult.ok) {
+          const [changeId, description] = parentResult.value.stdout
+            .trim()
+            .split("\t");
+          return ok({ changeId, description: description || "" });
+        }
+      }
+    }
+
+    // Navigate to the head of the stack
     const editResult = await this.run(["edit", "heads(descendants(@))"]);
     if (!editResult.ok) {
       if (editResult.error.message.includes("No descendant")) {
-        return err(createError("NAVIGATION_FAILED", "Already at top of stack"));
-      }
-      if (editResult.error.message.includes("more than one revision")) {
+        // Already at head - fall through to create empty @
+      } else if (editResult.error.message.includes("more than one revision")) {
         return err(
           createError(
             "NAVIGATION_FAILED",
             "Stack has multiple heads - cannot determine top",
           ),
         );
+      } else {
+        return editResult;
       }
-      return editResult;
     }
 
-    const status = await this.status();
-    if (!status.ok) return status;
+    // Create a new empty change above the head for new work
+    const newResult = await this.run(["new"]);
+    if (!newResult.ok) return newResult;
 
-    return ok({
-      changeId: status.value.workingCopy.changeId,
-      description: status.value.workingCopy.description,
-    });
+    // Get parent info (the actual stack top)
+    const parentResult = await this.run([
+      "log",
+      "-r",
+      "@-",
+      "--no-graph",
+      "-T",
+      'change_id.short() ++ "\\t" ++ description.first_line()',
+    ]);
+    if (!parentResult.ok) return parentResult;
+
+    const [changeId, description] = parentResult.value.stdout
+      .trim()
+      .split("\t");
+    return ok({ changeId, description: description || "" });
   }
 
   async navigateBottom(): Promise<Result<NavigationResult>> {