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/config/schema.ts

@@ -311,7 +311,6 @@ export const OhMyOpenCodeConfigSchema = z.object({
   agents: AgentOverridesSchema.optional(),
   categories: CategoriesConfigSchema.optional(),
   claude_code: ClaudeCodeConfigSchema.optional(),
-  google_auth: z.boolean().optional(),
   sisyphus_agent: SisyphusAgentConfigSchema.optional(),
   comment_checker: CommentCheckerConfigSchema.optional(),
   experimental: ExperimentalConfigSchema.optional(),