This skill fetches the agent card (JSON) from an A2A endpoint.
Input parameters (via stdin JSON):
url(string, required): The A2A endpoint URL to fetch the agent card from.
Output (stdout JSON):
ok(bool): Whether the fetch succeeded.card(string): The raw agent card JSON text (truncated to 2000 chars).source(string):"direct"or"well_known"indicating which fetch strategy succeeded.error(string): Error message if both attempts failed.