Extract RubyLLM::Prompt from Agent's private prompt rendering

[kryzhovnik]

Moves prompt root resolution, path construction, and ERB rendering into a standalone RubyLLM::Prompt class so prompts can be rendered without subclassing Agent.

Closes #667

What this does

Extracts RubyLLM::Prompt from Agent's private prompt_path_for and prompt_root methods. This lets users render prompts following the gem's own app/prompts/ convention without subclassing Agent:

RubyLLM::Prompt.render('friend', name: 'Andrey')

Agent's render_prompt now delegates to Prompt, preserving its existing class-name scoping and runtime context resolution.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Performance improvement

Scope check

• I read the Contributing Guide
• This aligns with RubyLLM's focus on LLM communication
• This isn't application-specific logic that belongs in user code
• This benefits most users, not just my specific use case

Required for new features

• I opened an issue before writing code and received maintainer approval
• Linked issue: [FEATURE] Extract RubyLLM::Prompt from Agent's existing prompt rendering internals #667

PRs for new features or enhancements without a prior approved issue will be closed.

Quality check

• I ran overcommit --install and all hooks pass
• I tested my changes thoroughly
  • For provider changes: Re-recorded VCR cassettes with bundle exec rake vcr:record[provider_name]
  • All tests pass: bundle exec rspec
• I updated documentation if needed
• I didn't modify auto-generated files manually (models.json, aliases.json)

AI-generated code

• I used AI tools to help write this code
• I have reviewed and understand all generated code (required if above is checked)

API changes

• Breaking change
• New public methods/classes
• Changed method signatures
• No API changes