Add Google OAuth login and native Gemini Code Assist support

[bdwelle]

Summary

• add a gambit auth google flow with PKCE, local callback server, and persisted tokens
• switch the Gemini provider to use OAuth tokens with the Code Assist endpoint, falling back to API key mode when available
• document setup instructions, add install script, and expose the auth command in the CLI docs

Testing

• deno run -A jsr:@bolt-foundry/gambit/cli auth google
• deno run -A jsr:@bolt-foundry/gambit/cli run ./storyshell.deck.md --message "echo hello" --stream