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/hook-config-handler.ts

@@ -4,8 +4,9 @@ import { log } from "../shared"
 
 export function applyHookConfig(params: {
   pluginComponents: PluginComponents;
+  ctx: { directory: string };
 }): void {
-  const { pluginComponents } = params
+  const { pluginComponents, ctx } = params
 
   if (pluginComponents.hooksConfigs.length > 0) {
     log("[hook-config-handler] Merging plugin hooks configs", {
@@ -14,5 +15,5 @@ export function applyHookConfig(params: {
     })
   }
 
-  setPluginHooksConfigs(pluginComponents.hooksConfigs)
+  setPluginHooksConfigs(ctx.directory, pluginComponents.hooksConfigs)
 }