feat: add Roast::Log class for standardized logging

[DerekStride]

Summary

• Introduces Roast::Log module as the central logging interface (addresses Roast::Log instead of Roast::Helpers::Logger #390)
• Outputs to $stderr by default so the logs can be redirected separate from workflow output, it's also testable via Minitest's capture_io
• Supports custom loggers via Roast::Log.logger = custom_logger
• Respects ROAST_LOG_LEVEL env var for log level configuration
• Updates claude_invocation.rb to use Roast::Log for unhandled message warnings

🤖 Generated with Claude Code

[mathiusj]

This lgtm. There are a couple other files that could benefit from using this logger, but not opposed to following up with those code changes 🙏 or if you're open to adding them to this PR, that would be great!

[nfgrep]

LGTM

[DerekStride]

I added another commit (dca0250) to update some of the obvious places that would fall clearly under logs / stderr output in my opinion.

There are other places like the following that I thought might fall under output / stdout which I didn't update. I thought that having the config.show_prompt! / config.show_response! / config.show_stats! conflict with ROAST_LOG_LEVEL would result in a confusing user experience. However, I'm happy to update those if you think they should go through the Log class. Alternatively, we can expose a Log#show_prompt and similar methods that respect the config & possibly go through a logger with a different formatter. I think that can be addressed in a separate PR if there's interest though.

roast/lib/roast/cogs/chat.rb

Lines 44 to 47 in 5446990

	when :user
	puts "[USER PROMPT] #{message.content}" if config.show_prompt?
	when :assistant
	puts "[LLM RESPONSE] #{message.content}" if config.show_response?

[dersam]

Let's keep the config options as they are - using the logger doesn't feel right. Logs are informational, while the config options are workflow specific. I don't think they should be beholden to log level.