fix(google-auth): integrate into main package via config option

Fixes #30. OpenCode's plugin loader treats subpath exports like
"oh-my-opencode/google-auth" as separate npm packages, causing
BunInstallFailedError.

Solution: Enable Google auth via `google_auth: true` in
oh-my-opencode.json instead of a separate subpath plugin.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/config/schema.ts

@@ -64,6 +64,7 @@ export const OhMyOpenCodeConfigSchema = z.object({
   disabled_agents: z.array(AgentNameSchema).optional(),
   agents: AgentOverridesSchema.optional(),
   claude_code: ClaudeCodeConfigSchema.optional(),
+  google_auth: z.boolean().optional(),
 })
 
 export type OhMyOpenCodeConfig = z.infer<typeof OhMyOpenCodeConfigSchema>