test+docs: add capability flag tests and documentation

Tests:
- adapter-registry: verify capability flags are exposed, undefined when
  unset, and correctly declared on built-in claude_local adapter
- adapter-routes: verify GET /api/adapters includes capabilities object,
  correct flags for codex_local/process/cursor, and derived supportsSkills

Documentation:
- docs/adapters/creating-an-adapter.md: new "Capability Flags" section
  documenting all flags, their defaults, and usage example for external
  adapter plugins

Co-Authored-By: Paperclip <noreply@paperclip.ing>

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

@@ -95,6 +95,51 @@ describe("server adapter registry", () => {
     ]);
   });
 
+  it("exposes capability flags from registered adapters", () => {
+    const adapterWithCaps: ServerAdapterModule = {
+      type: "external_test",
+      execute: async () => ({ exitCode: 0, signal: null, timedOut: false }),
+      testEnvironment: async () => ({
+        adapterType: "external_test",
+        status: "pass" as const,
+        checks: [],
+        testedAt: new Date(0).toISOString(),
+      }),
+      supportsLocalAgentJwt: true,
+      supportsInstructionsBundle: true,
+      instructionsPathKey: "customPathKey",
+      requiresMaterializedRuntimeSkills: true,
+    };
+
+    registerServerAdapter(adapterWithCaps);
+
+    const resolved = findActiveServerAdapter("external_test");
+    expect(resolved).not.toBeNull();
+    expect(resolved!.supportsInstructionsBundle).toBe(true);
+    expect(resolved!.instructionsPathKey).toBe("customPathKey");
+    expect(resolved!.requiresMaterializedRuntimeSkills).toBe(true);
+    expect(resolved!.supportsLocalAgentJwt).toBe(true);
+  });
+
+  it("returns undefined for capability flags on adapters that do not set them", () => {
+    registerServerAdapter(externalAdapter);
+
+    const resolved = findActiveServerAdapter("external_test");
+    expect(resolved).not.toBeNull();
+    expect(resolved!.supportsInstructionsBundle).toBeUndefined();
+    expect(resolved!.instructionsPathKey).toBeUndefined();
+    expect(resolved!.requiresMaterializedRuntimeSkills).toBeUndefined();
+  });
+
+  it("built-in claude_local adapter declares capability flags", () => {
+    const adapter = findActiveServerAdapter("claude_local");
+    expect(adapter).not.toBeNull();
+    expect(adapter!.supportsInstructionsBundle).toBe(true);
+    expect(adapter!.instructionsPathKey).toBe("instructionsFilePath");
+    expect(adapter!.requiresMaterializedRuntimeSkills).toBe(false);
+    expect(adapter!.supportsLocalAgentJwt).toBe(true);
+  });
+
   it("switches active adapter behavior back to the builtin when an override is paused", async () => {
     const builtIn = findServerAdapter("claude_local");
     expect(builtIn).not.toBeNull();

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

@@ -57,6 +57,75 @@ describe("adapter routes", () => {
     unregisterServerAdapter("claude_local");
   });
 
+  it("GET /api/adapters includes capabilities object for each adapter", async () => {
+    const app = createApp();
+
+    const res = await request(app).get("/api/adapters");
+    expect(res.status).toBe(200);
+    expect(Array.isArray(res.body)).toBe(true);
+    expect(res.body.length).toBeGreaterThan(0);
+
+    // Every adapter should have a capabilities object
+    for (const adapter of res.body) {
+      expect(adapter.capabilities).toBeDefined();
+      expect(typeof adapter.capabilities.supportsInstructionsBundle).toBe("boolean");
+      expect(typeof adapter.capabilities.supportsSkills).toBe("boolean");
+      expect(typeof adapter.capabilities.supportsLocalAgentJwt).toBe("boolean");
+      expect(typeof adapter.capabilities.requiresMaterializedRuntimeSkills).toBe("boolean");
+    }
+  });
+
+  it("GET /api/adapters returns correct capabilities for built-in adapters", async () => {
+    const app = createApp();
+
+    const res = await request(app).get("/api/adapters");
+    expect(res.status).toBe(200);
+
+    // codex_local has instructions bundle + skills + jwt, no materialized skills
+    // (claude_local is overridden by beforeEach, so check codex_local instead)
+    const codexLocal = res.body.find((a: any) => a.type === "codex_local");
+    expect(codexLocal).toBeDefined();
+    expect(codexLocal.capabilities).toMatchObject({
+      supportsInstructionsBundle: true,
+      supportsSkills: true,
+      supportsLocalAgentJwt: true,
+      requiresMaterializedRuntimeSkills: false,
+    });
+
+    // process adapter should have no local capabilities
+    const processAdapter = res.body.find((a: any) => a.type === "process");
+    expect(processAdapter).toBeDefined();
+    expect(processAdapter.capabilities).toMatchObject({
+      supportsInstructionsBundle: false,
+      supportsSkills: false,
+      supportsLocalAgentJwt: false,
+      requiresMaterializedRuntimeSkills: false,
+    });
+
+    // cursor adapter should require materialized runtime skills
+    const cursorAdapter = res.body.find((a: any) => a.type === "cursor");
+    expect(cursorAdapter).toBeDefined();
+    expect(cursorAdapter.capabilities.requiresMaterializedRuntimeSkills).toBe(true);
+    expect(cursorAdapter.capabilities.supportsInstructionsBundle).toBe(true);
+  });
+
+  it("GET /api/adapters derives supportsSkills from listSkills/syncSkills presence", async () => {
+    const app = createApp();
+
+    const res = await request(app).get("/api/adapters");
+    expect(res.status).toBe(200);
+
+    // http adapter has no listSkills/syncSkills
+    const httpAdapter = res.body.find((a: any) => a.type === "http");
+    expect(httpAdapter).toBeDefined();
+    expect(httpAdapter.capabilities.supportsSkills).toBe(false);
+
+    // codex_local has listSkills/syncSkills
+    const codexLocal = res.body.find((a: any) => a.type === "codex_local");
+    expect(codexLocal).toBeDefined();
+    expect(codexLocal.capabilities.supportsSkills).toBe(true);
+  });
+
   it("uses the active adapter when resolving config schema for a paused builtin override", async () => {
     const app = createApp();