feature: If commands such as `agents list` return 401, tell user to try `rossoctl login`

[esnible]

Feature Description

I'm always frustrated when rossoctl agents list outputs

Error: http://rossoctl-ui.localtest.me:8080/api/v1/agents?namespace=team1 returned 401: {"detail":"Token signing key not found"}

Proposed Solution

Instead of, or in addition to, the error message, we should suggest login for 401 errors.

The a2aSendCmd() function used by the rossoctl a2a send command prints the message "run rossoctl login to sign in", the agents and tools subcommands should print similar messages.

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Additional Context

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