feat(helper): add interactive Q&A helper command

Add `markitect helper <QUESTION>` CLI command that answers questions
about markitect using its own documentation as LLM context. Uses
OpenRouter with openrouter/aurora-alpha by default; model is
configurable via --model flag or MARKITECT_HELPER_MODEL env var.

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

markitect/cli.py

@@ -7095,6 +7095,13 @@ try:
 except ImportError:
     pass  # Prompts module not available
 
+# Register helper Q&A command
+try:
+    from markitect.helper.cli import helper_command
+    cli.add_command(helper_command)
+except ImportError:
+    pass  # Helper module not available
+
 # Make cli function available as main entry point
 main = cli