feat(cli): add 'helper' alias for markitect helper command

markitect helper <QUESTION> now works as a short alias for
markitect llm-helper, per the original plan specification.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

markitect/cli.py

@@ -7139,7 +7139,8 @@ try:
         helper_command, llm_catalog, llm_check,
         llm_default_command, llm_preference_command,
     )
-    cli.add_command(helper_command)
+    cli.add_command(helper_command)  # registers as llm-helper
+    cli.add_command(helper_command, name="helper")  # short alias
     cli.add_command(llm_catalog)
     cli.add_command(llm_check)
     cli.add_command(llm_default_command)