fix: sync README config examples and make Daytona optional

[Caldalis]

Summary

Sync the configuration examples in all README files with the current default config/config.example.toml, and make Daytona configuration optional
when no [daytona] section is provided.

The READMEs still showed the old OpenAI gpt-4o example, while the default config now uses Anthropic Claude. Also, loading the default config
could fail because DaytonaSettings.daytona_api_key was required even though config.example.toml does not include Daytona settings.

Changes

• Updated configuration snippets in:
  • README.md
  • README_zh.md
  • README_ko.md
  • README_ja.md
• Added references to all provider-specific config templates in config/
• Set DaytonaSettings.daytona_api_key default to an empty string

Validation

• Ran git diff --check
• Ran python -m py_compile app\config.py
• Verified default config loads successfully from config/config.example.toml
• Verified README files no longer contain the old OpenAI gpt-4o config snippet