curl -X POST "http://localhost:8000/api/v2/agents/my-agent/policy/preset" \
  -H "X-Session-Token: your_session_token" \
  -H "Content-Type: application/json" \
  -d '{"name": "balanced"}'

{
  "agent_id": "my-agent",
  "preset": "balanced",
  "policy": {
    "version": 1,
    "retention": { "context": "7d", "event": "30d", "error": "30d" },
    "rules": [
      { "name": "pinned", "match": { "tags": ["pinned"] }, "expire_after": "never" }
    ],
    "purge_expired_after": "never"
  }
}

{
  "detail": "Unknown preset 'nope'. Must be one of: aggressive, balanced, conservative."
}

{
  "detail": "Missing session token. Use X-Session-Token header."
}