feat: enhance merge workflow by detecting current branch

- Added functionality to detect the current Git branch before merging spec changes.
- Prevent merging into the same branch by providing user guidance to switch branches.
- Updated WorktreeManager initialization to use the detected branch as the merge target.

This improves the user experience by ensuring that merges are performed correctly and reduces the risk of accidental merges into the spec branch.

auto-claude/core/workspace.py

@@ -164,6 +164,35 @@ def merge_existing_build(
         print(highlight(f"  python auto-claude/run.py --spec {spec_name}"))
         return False
 
+    # Detect current branch - this is where user wants changes merged
+    # Normal workflow: user is on their feature branch (e.g., version/2.5.5)
+    # and wants to merge the spec changes into it, then PR to main
+    current_branch_result = subprocess.run(
+        ["git", "rev-parse", "--abbrev-ref", "HEAD"],
+        cwd=project_dir,
+        capture_output=True,
+        text=True,
+    )
+    current_branch = (
+        current_branch_result.stdout.strip()
+        if current_branch_result.returncode == 0
+        else None
+    )
+
+    spec_branch = f"auto-claude/{spec_name}"
+
+    # Don't merge a branch into itself
+    if current_branch == spec_branch:
+        print()
+        print_status(
+            "You're on the spec branch. Switch to your target branch first.", "warning"
+        )
+        print()
+        print("Example:")
+        print(highlight(f"  git checkout main  # or your feature branch"))
+        print(highlight(f"  python auto-claude/run.py --spec {spec_name} --merge"))
+        return False
+
     if no_commit:
         content = [
             bold(f"{icon(Icons.SUCCESS)} STAGING BUILD FOR REVIEW"),
@@ -178,7 +207,8 @@ def merge_existing_build(
     print()
     print(box(content, width=60, style="heavy"))
 
-    manager = WorktreeManager(project_dir)
+    # Use current branch as merge target (not auto-detected main/master)
+    manager = WorktreeManager(project_dir, base_branch=current_branch)
     show_build_summary(manager, spec_name)
     print()