feat(plugin): Add Claude Code plugin management system

- Add plugin management commands using claude CLI directly
- Support marketplace operations (add, remove, list, update)
- Support plugin operations (install, uninstall, enable, disable, validate)
- Add built-in plugin repository (compounding-engineering)
- Refactored architecture with base handler class for future extensibility

Commands:
- cam plugin install/uninstall/enable/disable <plugin>
- cam plugin list [--all]
- cam plugin repos
- cam plugin info
- cam plugin validate <path>
- cam plugin marketplace add/remove/list/update

Author: zhujian0805

code_assistant_manager/cli/app.py

@@ -8,6 +8,7 @@
 import typer
 from typer import Context
 
+from code_assistant_manager.cli.plugin_commands import plugin_app
 from code_assistant_manager.cli.prompts_commands import prompt_app
 from code_assistant_manager.cli.skills_commands import skill_app
 from code_assistant_manager.config import ConfigManager
@@ -206,6 +207,9 @@ def command(
 # Add the skill app as a subcommand to the main app
 app.add_typer(skill_app, name="skill")
 app.add_typer(skill_app, name="s", hidden=True)
+# Add the plugin app as a subcommand to the main app (Claude Code plugins)
+app.add_typer(plugin_app, name="plugin")
+app.add_typer(plugin_app, name="pl", hidden=True)
 
 
 @config_app.command("validate")