feat: add Grok (xAI) API adapter with custom model mapping (#152)

Add xAI Grok as a new API provider. Reuses OpenAI-compatible message/tool
converters and stream adapter with Grok-specific client and model mapping.

Default model mapping:
  opus   → grok-4.20-reasoning
  sonnet → grok-3-mini-fast
  haiku  → grok-3-mini-fast

Users can customize mapping via:
  - GROK_MODEL env var (override all)
  - GROK_MODEL_MAP env var (JSON family map, e.g. {"opus":"grok-4"})
  - GROK_DEFAULT_{FAMILY}_MODEL env vars

Activation: CLAUDE_CODE_USE_GROK=1 or modelType: "grok" in settings.json
Also integrates with /provider command for runtime switching.

src/services/api/grok/__tests__/client.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, test, beforeEach, afterEach } from 'bun:test'
+import { getGrokClient, clearGrokClientCache } from '../client.js'
+
+describe('getGrokClient', () => {
+  const originalEnv = { ...process.env }
+
+  beforeEach(() => {
+    clearGrokClientCache()
+    process.env.GROK_API_KEY = 'test-key'
+    delete process.env.GROK_BASE_URL
+  })
+
+  afterEach(() => {
+    clearGrokClientCache()
+    process.env = { ...originalEnv }
+  })
+
+  test('creates client with default base URL', () => {
+    const client = getGrokClient()
+    expect(client).toBeDefined()
+    expect(client.baseURL).toBe('https://api.x.ai/v1')
+  })
+
+  test('uses GROK_BASE_URL when set', () => {
+    process.env.GROK_BASE_URL = 'https://custom.grok.api/v1'
+    clearGrokClientCache()
+    const client = getGrokClient()
+    expect(client.baseURL).toBe('https://custom.grok.api/v1')
+  })
+
+  test('returns cached client on second call', () => {
+    const client1 = getGrokClient()
+    const client2 = getGrokClient()
+    expect(client1).toBe(client2)
+  })
+
+  test('clearGrokClientCache resets cache', () => {
+    const client1 = getGrokClient()
+    clearGrokClientCache()
+    process.env.GROK_BASE_URL = 'https://other.api/v1'
+    const client2 = getGrokClient()
+    expect(client1).not.toBe(client2)
+  })
+})