✨ Add Visitor pattern support to AST

[sakuro]

Summary

Add Visitor pattern support to foxtail-tools AST for reusable tree traversal without ad-hoc case statements.

Changes

• Add Foxtail::Syntax::Visitor module with visit_* hooks for all 23 AST node types
• Add accept(visitor) and children methods to AST nodes
• Default visit_* methods auto-traverse children; omit super to stop traversal
• Refactor example to demonstrate Visitor pattern usage

Test Plan

bundle exec rake spec:tools
bundle exec rake rubocop:tools
cd foxtail-tools && bundle exec ruby examples/01_prefix_message_ids.rb

Closes #155