feat(adapters): external adapter plugin system with dynamic UI parser

- Plugin loader: install/reload/remove/reinstall external adapters
  from npm packages or local directories
- Plugin store persisted at ~/.paperclip/adapter-plugins.json
- Self-healing UI parser resolution with version caching
- UI: Adapter Manager page, dynamic loader, display registry
  with humanized names for unknown adapter types
- Dev watch: exclude adapter-plugins dir from tsx watcher
  to prevent mid-request server restarts during reinstall
- All consumer fallbacks use getAdapterLabel() for consistent display
- AdapterTypeDropdown uses controlled open state for proper close behavior
- Remove hermes-local from built-in UI (externalized to plugin)
- Add docs for external adapters and UI parser contract

server/src/__tests__/adapter-registry.test.ts

@@ -0,0 +1,58 @@
+import { describe, expect, it, beforeEach, afterEach } from "vitest";
+import type { ServerAdapterModule } from "../adapters/index.js";
+import {
+  findServerAdapter,
+  listAdapterModels,
+  registerServerAdapter,
+  requireServerAdapter,
+  unregisterServerAdapter,
+} from "../adapters/index.js";
+
+const externalAdapter: ServerAdapterModule = {
+  type: "external_test",
+  execute: async () => ({
+    exitCode: 0,
+    signal: null,
+    timedOut: false,
+  }),
+  testEnvironment: async () => ({
+    adapterType: "external_test",
+    status: "pass",
+    checks: [],
+    testedAt: new Date(0).toISOString(),
+  }),
+  models: [{ id: "external-model", label: "External Model" }],
+  supportsLocalAgentJwt: false,
+};
+
+describe("server adapter registry", () => {
+  beforeEach(() => {
+    unregisterServerAdapter("external_test");
+  });
+
+  afterEach(() => {
+    unregisterServerAdapter("external_test");
+  });
+
+  it("registers external adapters and exposes them through lookup helpers", async () => {
+    expect(findServerAdapter("external_test")).toBeNull();
+
+    registerServerAdapter(externalAdapter);
+
+    expect(requireServerAdapter("external_test")).toBe(externalAdapter);
+    expect(await listAdapterModels("external_test")).toEqual([
+      { id: "external-model", label: "External Model" },
+    ]);
+  });
+
+  it("removes external adapters when unregistered", () => {
+    registerServerAdapter(externalAdapter);
+
+    unregisterServerAdapter("external_test");
+
+    expect(findServerAdapter("external_test")).toBeNull();
+    expect(() => requireServerAdapter("external_test")).toThrow(
+      "Unknown adapter type: external_test",
+    );
+  });
+});