feat(server): add plugin kernel MVP with extensible server API

[Flussen]

Motivation

• Provide a simple, compile-time plugin kernel so libraries can integrate formally into the server runtime and register decorators, DI services and server API extensions.
• Allow safe, typeable server API extensions via TypeScript module augmentation while keeping existing runtime behavior and client bundle unchanged.

Description

• Added a small plugin kernel under src/runtime/core/plugin with OpenCorePlugin, PluginInstallContext and PluginRegistry that prevents duplicate installs (PluginRegistry.installAll).
• Introduced a server extension surface: src/runtime/server/plugin/server-plugin-api.ts (marker interface for augmentation), ServerApi typing, createServerRuntime() and registerApiExtension() runtime wiring in src/runtime/server/server.runtime.ts to attach extensions to the server surface.
• Integrated plugin installation into server bootstrap by extending init() with OpenCoreInitOptions.plugins?: OpenCorePlugin[] and installing plugins before initServer() using an install context exposing server, di.register (backs to GLOBAL_CONTAINER.registerInstance) and a config.get accessor in src/runtime/server/core.ts.
• Exported plugin surface from runtime/core/index.ts and updated runtime/server/index.ts exports, and added focused unit tests for installation ordering, duplicate detection, runtime extension behavior and module-augmentation typing.

Testing

• Ran the new unit tests with pnpm -s test:unit tests/unit/runtime/core/plugin-registry.test.ts tests/unit/runtime/core/plugin-typing.test.ts and both tests passed.
• Ran type checking with pnpm -s typecheck and confirmed the final run passed after adjusting runtime typing surface to avoid type errors.
• Built the package with pnpm -s build and the build completed successfully.

---

Codex Task