Add social card (og:image) for link previews

[skokaina]

Why

When the repo or docs URL is shared on Twitter/Reddit/Slack/Discord, the link preview card is the first thing people see. A clean og:image with the logo and one-liner converts impressions into clicks.

Tasks

GitHub repo social preview

• Create a 1280x640 image: logo + tagline ("One agent.yaml. Validate, health-check, audit, generate.")
• Set it in repo Settings → Social preview

Docs site (VitePress)

• Add og:image meta tag in `docs/.vitepress/config.mts`:
  ```ts
  head: [
  ['meta', { property: 'og:image', content: 'https://agentspec.io/og-card.png' }],
  ['meta', { property: 'og:title', content: 'AgentSpec — Universal Agent Manifest System' }],
  ['meta', { property: 'og:description', content: 'One agent.yaml. Validate, health-check, audit, and generate any AI agent.' }],
  ]
  ```
• Add Twitter card meta tags (`twitter:card`, `twitter:image`)

Verification

• Test with https://www.opengraph.xyz/ or Twitter Card Validator
• Preview looks good on dark and light backgrounds

Acceptance criteria

• Sharing the GitHub repo URL shows a branded preview card
• Sharing the docs URL shows a branded preview card