Make rails-docs and tailwind-docs derive version dynamically

[mattlindsey]

Summary

• Follow-up to Add Rails official documentation skill #746 (already merged), which added rails-docs, hotwire-docs, and tailwind-docs skills.
• rails-docs and tailwind-docs hardcoded version-specific claims ("this project pins Rails 8.0" / "pins Tailwind v3") that would go stale silently the moment either dependency is bumped, with nothing to flag it.
• Both skills now instruct Claude to determine the actual version at query time instead:
  • rails-docs: greps Gemfile.lock for the installed rails version and builds the versioned guide/API URLs from that.
  • tailwind-docs: checks for config/tailwind.config.js (v3-style JS config) vs @theme in the stylesheets (v4-style CSS config) to determine which docs to consult.
• No functional app code touched — skill instructions only.

Test plan

• N/A — documentation/skill-only change, no app code touched.