Fetch an agent avatar image

Returns the raw image bytes for an agent's profile picture identified by file. This endpoint is designed for integration partners (such as Slack) that fetch avatar URLs via plain GET requests without bearer token support. Authorization is performed via a short, stable capability token rather than an HTTP header.

The token is an HMAC-based capability tied to the file ID. It does not expire, but it is invalidated when the agent's profile picture is replaced or the agent is deleted — shared caches may continue serving the old image until the Cache-Control max-age of one hour elapses. The endpoint never redirects to a signed storage URL; bytes are served inline so behavior is consistent across storage backends.

All failure modes — invalid file ID, invalid token, file not currently referenced as an agent avatar — return a uniform 404 to avoid acting as an existence oracle.