Refactor: Reorganize plugin system into proper package structure

- Move all plugin commands into code_assistant_manager/cli/plugins/ package
- Create proper __init__.py and organize plugin modules
- Fix marketplace vs plugin installation logic (add return after marketplace handling)
- Remove unwanted shortcut commands (ls, i, rm) as requested
- Add comprehensive MCP system modules and functionality
- Update all imports and test patches to use new package structure
- Add extensive test coverage for plugin system
- Fix UnboundLocalError in install_plugin function
- Ensure all 31 plugin tests pass
- Verify CLI functionality and marketplace/plugin distinction

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

Author: zhujian0805

code_assistant_manager/cli/plugin_commands.py

@@ -0,0 +1,7 @@
+"""Plugin management commands package.
+
+This package contains all plugin-related CLI commands organized into separate modules.
+"""
+
+# This package contains individual plugin command modules
+# The main plugin app is assembled in ../plugin_commands.py