Create an AI Agents & Skills page in the docs

Add a dedicated page covering:

• What the miren-skills are and what they do
• Installation for each supported agent (Claude Code, Codex, Amp, Pi, OpenCode)
• Which skills exist and when they help
• How skills and docs relate ("skills make the docs faster to act on, but the docs remain authoritative")
• Link to the skills repo

This is the page an agent retrieves when a user asks "does Miren have AI tooling?" and the page a human reads when deciding whether to install. It's also what llms.txt should prominently surface.

Impact: High for discoverability of the skills ecosystem.