refactor(claude-code-plugin-loader): split loader.ts into per-type loaders

Extract plugin component loading into dedicated modules:
- discovery.ts: plugin directory detection
- plugin-path-resolver.ts: path resolution logic
- agent-loader.ts, command-loader.ts, hook-loader.ts
- mcp-server-loader.ts, skill-loader.ts

src/features/claude-code-plugin-loader/index.ts

@@ -1,3 +1,10 @@
 export * from "./types"
 export * from "./loader"
+export * from "./discovery"
+export * from "./plugin-path-resolver"
+export * from "./command-loader"
+export * from "./skill-loader"
+export * from "./agent-loader"
+export * from "./mcp-server-loader"
+export * from "./hook-loader"
 export type { PluginLoaderOptions, ClaudeSettings } from "./types"