Fix: agents.json

Capability manifest. Prefer /.well-known/agents.json (also checked at /agents.json). Schemas in the wild still fragment; publish valid JSON describing what agents can do.

{
  "agentsJson": "1.0.0",
  "info": {
    "title": "Example Agent Surface",
    "version": "1.0.0",
    "description": "Describe tools your site exposes to agents"
  }
}