feat(agents): add isGeminiModel detection function with TDD

Detects Gemini models via:
- Provider prefixes: google/, google-vertex/
- GitHub Copilot: github-copilot/gemini-*
- Model name: gemini-* (for proxied providers like litellm)

Follows existing isGptModel pattern. All 16 tests pass.

src/agents/types.test.ts

@@ -1,5 +1,5 @@
 import { describe, test, expect } from "bun:test";
-import { isGptModel } from "./types";
+import { isGptModel, isGeminiModel } from "./types";
 
 describe("isGptModel", () => {
   test("standard openai provider models", () => {
@@ -47,3 +47,47 @@ describe("isGptModel", () => {
     expect(isGptModel("opencode/claude-opus-4-6")).toBe(false);
   });
 });
+
+describe("isGeminiModel", () => {
+  test("#given google provider models #then returns true", () => {
+    expect(isGeminiModel("google/gemini-3-pro")).toBe(true);
+    expect(isGeminiModel("google/gemini-3-flash")).toBe(true);
+    expect(isGeminiModel("google/gemini-2.5-pro")).toBe(true);
+  });
+
+  test("#given google-vertex provider models #then returns true", () => {
+    expect(isGeminiModel("google-vertex/gemini-3-pro")).toBe(true);
+    expect(isGeminiModel("google-vertex/gemini-3-flash")).toBe(true);
+  });
+
+  test("#given github copilot gemini models #then returns true", () => {
+    expect(isGeminiModel("github-copilot/gemini-3-pro")).toBe(true);
+    expect(isGeminiModel("github-copilot/gemini-3-flash")).toBe(true);
+  });
+
+  test("#given litellm proxied gemini models #then returns true", () => {
+    expect(isGeminiModel("litellm/gemini-3-pro")).toBe(true);
+    expect(isGeminiModel("litellm/gemini-3-flash")).toBe(true);
+    expect(isGeminiModel("litellm/gemini-2.5-pro")).toBe(true);
+  });
+
+  test("#given other proxied gemini models #then returns true", () => {
+    expect(isGeminiModel("custom-provider/gemini-3-pro")).toBe(true);
+    expect(isGeminiModel("ollama/gemini-3-flash")).toBe(true);
+  });
+
+  test("#given gpt models #then returns false", () => {
+    expect(isGeminiModel("openai/gpt-5.2")).toBe(false);
+    expect(isGeminiModel("openai/o3-mini")).toBe(false);
+    expect(isGeminiModel("litellm/gpt-4o")).toBe(false);
+  });
+
+  test("#given claude models #then returns false", () => {
+    expect(isGeminiModel("anthropic/claude-opus-4-6")).toBe(false);
+    expect(isGeminiModel("anthropic/claude-sonnet-4-6")).toBe(false);
+  });
+
+  test("#given opencode provider #then returns false", () => {
+    expect(isGeminiModel("opencode/claude-opus-4-6")).toBe(false);
+  });
+});