`swarm_queue_client::agent_token::format_agent_token` / `parse_agent_token`
are the spelling an agent presents its own queue secret in,
`swarm-agent.<agent>.<secret>`, and the one the auth-callout responder
reads back. The prefix is what separates it from an OIDC access token,
which may itself contain `.`. Parsing distinguishes "not an agent token"
(no prefix) from "a malformed one"; the error names the problem and never
the value. The module is store-free, so the agent formats its token
without linking the secret-store client.
`swarm_secret_client::queue::AgentCredential` loses `hive`: an agent's
identity is not tied to a hive, and nothing reads the field. Objects
already in the store carry it and still decode, since unknown fields are
ignored; a test parses one. The controller stops writing it.
With the credential no longer naming a hive, and the agent's policy
naming none since #4762, nothing in the mint consumes one. `hive` goes
from `mint_and_verify`, from the `MintAgentIdentity` node, and from
`POST /api/agents/{name}/identity`, which now takes no body and no longer
checks a hive against the roster; a caller that still sends one is not
refused, the body is ignored. `swarmctl agent mint-identity` loses
`--hive`, so passing it is now a usage error.
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