Register and configure agentspec.tech domain

[skokaina]

Why

The apiVersion already uses `agentspec.tech` throughout the codebase. The docs site is at `agents-oss.github.io/agentspec`. After an HN post, someone will squat the domain if it's not registered. A custom domain also signals project legitimacy.

Tasks

• Register `agentspec.tech`
• Configure DNS: point to GitHub Pages (`agents-oss.github.io`)
• Add CNAME file to docs deployment
• Update VitePress config (`docs/.vitepress/config.mts`) with new base URL
• Update README links from `agents-oss.github.io/agentspec` to `agentspec.io`
• Verify HTTPS works (GitHub Pages provides free cert for custom domains)