Add version guide and detection patterns for Rails 3.0 → 3.1 upgrade

[etagwerker]

The ebook ("The Complete Guide to Upgrade Rails", 5th Edition) covers Rails 3.0 → 3.1 in from-3-0-to-3-1.md, but the skill has no version guide or detection patterns for this upgrade path.

This upgrade introduces the Asset Pipeline, jQuery as default, and config file changes.

Source material: https://github.com/ombulabs/ebook/blob/main/from-3-0-to-3-1.md

Acceptance criteria

• Create version-guides/upgrade-3.0-to-3.1.md based on ebook content
• Create detection-scripts/patterns/rails-31-patterns.yml covering all breaking changes
• Verify YAML parses without errors
• Update SKILL.md supported upgrade paths table if needed