feat: add Gemini ADC and access token auth (#312)

* feat: add Gemini ADC and access token auth

* feat: add Gemini token and ADC provider setup

* feat: add Gemini token and ADC provider setup

* fix: honor Gemini auth mode on restart

src/utils/geminiCredentials.test.ts

@@ -0,0 +1,31 @@
+import { afterEach, expect, test } from 'bun:test'
+
+import {
+  clearGeminiAccessToken,
+  readGeminiAccessToken,
+  saveGeminiAccessToken,
+} from './geminiCredentials.ts'
+
+const originalToken = process.env.GEMINI_ACCESS_TOKEN
+
+afterEach(() => {
+  if (originalToken === undefined) {
+    delete process.env.GEMINI_ACCESS_TOKEN
+  } else {
+    process.env.GEMINI_ACCESS_TOKEN = originalToken
+  }
+  clearGeminiAccessToken()
+})
+
+test('saveGeminiAccessToken stores and reads back the token', () => {
+  const result = saveGeminiAccessToken('token-123')
+  expect(result.success).toBe(true)
+  expect(readGeminiAccessToken()).toBe('token-123')
+})
+
+test('clearGeminiAccessToken removes the stored token', () => {
+  expect(saveGeminiAccessToken('token-123').success).toBe(true)
+  expect(clearGeminiAccessToken().success).toBe(true)
+  expect(readGeminiAccessToken()).toBeUndefined()
+})
+