Build documentation and agents file

[lomefin]

Summary

• Added developer-facing docs with a focused guide in docs/DEVELOPERS.md and linked it from README.md.
• Introduced an AGENTS.md file to capture repo conventions, commands, and guidance for automated tooling.
• Fixed block forwarding in callback alias methods: validates_with, prepares_with, and done_criteria.
• Added/updated specs to cover alias behavior, after_init callbacks, and check with multiple callbacks.
• Added .mise.toml to pin Ruby 3.1.6 and updated Gemfile.lock from running Bundler.

Key Files

• docs/DEVELOPERS.md
• AGENTS.md
• README.md
• lib/storyteller.rb
• spec/storyteller_spec.rb
• .mise.toml
• Gemfile.lock

Tests

• mise exec -- bundle exec rake spec