refactor: remove built-in Google auth in favor of external plugin

- Delete src/auth/antigravity/ directory (28 files)
- Delete src/google-auth.ts standalone wrapper
- Delete src/cli/commands/auth.ts CLI command
- Remove google_auth config option from schema
- Update CLI to remove auth command registration
- Update config-manager to remove google_auth handling
- Update documentation to reference external opencode-antigravity-auth plugin only
- Regenerate JSON schema

Users should install the opencode-antigravity-auth plugin for Gemini authentication.

BREAKING CHANGE: The google_auth config option is removed. Use the external plugin instead.

src/index.ts

@@ -36,7 +36,6 @@ import {
   createContextInjectorHook,
   createContextInjectorMessagesTransformHook,
 } from "./features/context-injector";
-import { createGoogleAntigravityAuthPlugin } from "./auth/antigravity";
 import { applyAgentVariant, resolveAgentVariant } from "./shared/agent-variant";
 import { createFirstMessageVariantGate } from "./shared/first-message-variant";
 import {
@@ -293,10 +292,6 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
     ? createAutoSlashCommandHook({ skills: mergedSkills })
     : null;
 
-  const googleAuthHooks = pluginConfig.google_auth !== false
-    ? await createGoogleAntigravityAuthPlugin(ctx)
-    : null;
-
   const configHandler = createConfigHandler({
     ctx,
     pluginConfig,
@@ -304,8 +299,6 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
   });
 
   return {
-    ...(googleAuthHooks ? { auth: googleAuthHooks.auth } : {}),
-
     tool: {
       ...builtinTools,
       ...backgroundTools,