Add AI Agent Skills and Knowledge category

[serradura]

Adds a category for gems that package knowledge and reusable skills for AI coding agents to read. Four gems qualify today, so it clears the two-entry bar:

• agent_skill_parser parses skill files (YAML frontmatter plus Markdown body).
• bundler-skills finds skills/ directories inside your dependency gems and links them into the project's agent skills directory.
• ruby_llm-skills loads and validates Agent Skills for RubyLLM.
• okf reads, validates, lints, searches, and serves Open Knowledge Format bundles, the Markdown plus YAML format an agent reads project knowledge from. I maintain this one, which is what prompted the PR, but it is not the reason for the category.

I put it under Machine Learning because that is the closest existing group. Developer Tools is arguable, and a separate AI group would be another option if more of these land. Rename or move it as you see fit.

Checklist from the template: all four are published gems, referenced by gem name rather than GitHub path.

I could not run the suite locally since my Ruby is 4.0.5 and .ruby-version pins 3.4.1. Instead I checked the three things the specs assert, by loading the whole catalog the way lib/catalog.rb does: the shape validates against json-schema.yml (30 groups, 219 categories, permalinks and project names matching their patterns), the new category has no duplicates, and all four gem names resolve on RubyGems.