feat: add plugin_marketplaces input for dynamic marketplace installation (#642)

- Added plugin_marketplaces input to both main and base-action action.yml files
- Updated install-plugins.ts to support multiple marketplace URLs (newline-separated)
- Added validation for marketplace URLs to prevent security issues
- Updated installPlugins function to dynamically add marketplaces instead of hardcoding
- Defaults to official Claude Code marketplace when no marketplaces are specified
- Updated base-action index.ts to pass plugin_marketplaces to installPlugins

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

base-action/src/index.ts

@@ -20,6 +20,7 @@ async function run() {
     await installPlugins(
       process.env.INPUT_PLUGINS,
       process.env.INPUT_PATH_TO_CLAUDE_CODE_EXECUTABLE,
+      process.env.INPUT_PLUGIN_MARKETPLACES,
     );
 
     const promptConfig = await preparePrompt({