fix: address PR #4180 review - security, typing, and test coverage

- Apply mcp_env_allowlist to plugin hooks: intersect HTTP allowedEnvVars
  with MCP allowlist, set command allowedEnvVars to full MCP allowlist
- Scrub process.env in executeHookCommand when allowedEnvVars provided
- Add PluginHooksState class with per-directory Map storage
- Add PluginHooksConfig interface for typed boundary layer
- Pass directory context through hook-config-handler
- Add 16 tests across 4 files (40 assertions) covering allowlist
  filtering, env scrubbing, directory isolation, and edge cases
- Remove unnecessary 'as' type assertions, use discriminated union
  narrowing instead

src/plugin-handlers/config-handler.ts

@@ -31,7 +31,7 @@ export function createConfigHandler(deps: ConfigHandlerDeps) {
 
     const pluginComponents = await loadPluginComponents({ pluginConfig });
 
-    applyHookConfig({ pluginComponents });
+    applyHookConfig({ pluginComponents, ctx });
 
     const agentResult = await applyAgentConfig({
       config,