When spawning an agent via the API, a relative webhook URL like '/webhook' is accepted but later causes 'InvalidUrlException "/webhook" "Invalid URL"'. Validate that the webhook field is a full URL (http/https) at spawn time and return a 400 error if it's not. This prevents confusing errors later in the agent lifecycle.
Ava verified: cited commit confirmed in live history; feature shipped. Moving to Verified.
Fixed in b697c45: validateWebhookUrl checks http(s):// prefix. spawnHandler returns 400 for invalid URLs. spawnPiAgent also validates and records failure.