Pin Ruby version to 3.1.0 via asdf

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: thiagoa

.gitignore

@@ -1,3 +1,4 @@
+!.tool-versions
 /.bundle/
 /.yardoc
 /_yardoc/

.ruby-version

@@ -0,0 +1 @@
+3.1.0

.tool-versions

@@ -0,0 +1 @@
+ruby 3.1.0

CLAUDE.md

@@ -15,6 +15,19 @@
 - After releasing, delete the generated `.gem` files from the root
   directory — they are not committed and should not linger.
 
+## Ruby version
+
+For now, use the minimum required Ruby version for development —
+this ensures the gem works for everyone who meets that requirement.
+
+When updating the minimum required Ruby version, update it in all
+of these places:
+
+- `.ruby-version`
+- `.tool-versions`
+- `duck_typer.gemspec` (`spec.required_ruby_version`)
+- `.github/workflows/ci.yml` (`ruby-version`)
+
 ## Commands
 
 - `bundle exec rake test` — run Minitest and RSpec suites