fix(export): use python -m nbdev.export instead of nbdev_export command

The nbdev_export command wasn't in PATH in CI. Using sys.executable
with -m nbdev.export ensures we use the same Python environment that's
running tito, which is more reliable across different environments.

Fixed in:
- tito/commands/dev/export.py (both _export_all_modules and _run_nbdev_export)
- tito/commands/module/workflow.py (_export_notebook)

tinytorch/tito/commands/dev/export.py

@@ -208,8 +208,10 @@ class DevExportCommand(BaseCommand):
         # Step 2: Export all using nbdev_export
         console.print("🔄 Exporting all notebooks to tinytorch package...")
         try:
+            # Use sys.executable to ensure we use the same Python that's running tito
+            import sys
             result = subprocess.run(
-                ["nbdev_export"],
+                [sys.executable, "-m", "nbdev.export"],
                 capture_output=True,
                 text=True,
                 cwd=Path.cwd()
@@ -240,6 +242,7 @@ class DevExportCommand(BaseCommand):
 
     def _run_nbdev_export(self, notebook_paths: list, console) -> int:
         """Run nbdev_export on the given notebooks."""
+        import sys
         success_count = 0
 
         for notebook_path_str in notebook_paths:
@@ -254,8 +257,9 @@ class DevExportCommand(BaseCommand):
                 if export_target != "unknown":
                     ensure_writable_target(export_target)
 
+                # Use sys.executable to ensure we use the same Python that's running tito
                 result = subprocess.run(
-                    ["nbdev_export", "--path", notebook_path_str],
+                    [sys.executable, "-m", "nbdev.export", "--path", notebook_path_str],
                     capture_output=True,
                     text=True,
                     cwd=Path.cwd()

tinytorch/tito/commands/module/workflow.py

@@ -1110,10 +1110,12 @@ class ModuleWorkflowCommand(BaseCommand):
                 ensure_writable_target(export_target)
             
             # Run nbdev_export directly on the student's notebook
+            # Use sys.executable to ensure we use the same Python that's running tito
+            import sys
             self.console.print(f"[dim]📦 Exporting {notebook_path.name} → tinytorch/core/...[/dim]")
             
             result = subprocess.run(
-                ["nbdev_export", "--path", str(notebook_path)],
+                [sys.executable, "-m", "nbdev.export", "--path", str(notebook_path)],
                 capture_output=True,
                 text=True,
                 cwd=Path.cwd()