docs(modules): add client manifest generation and update discovery docs

- introduce `OUTPUT_CLIENT` constant and `renderClientManifest` for `'use client'` bundle
- rename `renderManifest` to `renderServerManifest` for clarity
- update `sync` command to write both server and client manifests
- update `findInstalledModuleNames` to support custom package path resolution
- rewrite MODULES.md to explain dual-manifest architecture and client hydration rationale

src/core/modules/discover.server.js

@@ -1,7 +1,7 @@
 import { readFile } from 'node:fs/promises';
 import { resolve, join } from 'node:path';
 import { createRequire } from 'node:module';
-import { warn } from '@zen/core/shared/logger';
+import { info, warn } from '@zen/core/shared/logger';
 import { registerModule } from './registry.js';
 
 /**
@@ -43,6 +43,7 @@ export function registerModules(modules) {
       createTables: ex.createTables,
       dropTables: ex.dropTables,
     });
+    info(`zen-modules: registered ${ex.manifest.name}@${ex.manifest.version ?? '?'}`);
   }
 }