feat(librarian): conditionally enable thinking based on model type

- Add isGeminiModel helper to detect Gemini models
- Disable thinking config for Gemini models (not supported)
- Enable thinking with 32000 token budget for other models
- Add tests verifying both Gemini and Claude behavior

🤖 Generated with assistance of OhMyOpenCode

src/agents/utils.test.ts

@@ -91,6 +91,34 @@ describe("createBuiltinAgents with model overrides", () => {
     expect(agents.oracle.textVerbosity).toBeUndefined()
   })
 
+  test("Librarian with Gemini model has no thinking", async () => {
+    // #given
+    const overrides = {
+      librarian: { model: "google/gemini-3-flash-preview" },
+    }
+
+    // #when
+    const agents = await createBuiltinAgents([], overrides, undefined, TEST_DEFAULT_MODEL)
+
+    // #then
+    expect(agents.librarian.model).toBe("google/gemini-3-flash-preview")
+    expect(agents.librarian.thinking).toBeUndefined()
+  })
+
+  test("Librarian with Claude model has thinking enabled", async () => {
+    // #given
+    const overrides = {
+      librarian: { model: "anthropic/claude-sonnet-4-5" },
+    }
+
+    // #when
+    const agents = await createBuiltinAgents([], overrides, undefined, TEST_DEFAULT_MODEL)
+
+    // #then
+    expect(agents.librarian.model).toBe("anthropic/claude-sonnet-4-5")
+    expect(agents.librarian.thinking).toEqual({ type: "enabled", budgetTokens: 32000 })
+  })
+
    test("non-model overrides are still applied after factory rebuild", async () => {
      // #given
      const overrides = {