The agent's turn-state header ($SWARM.agent-state.<agent>) gains
`needs_login`, set by hive-agent from its own LoginState, so the flag
reaches the swarm without hive-c0re (mara on #5268: "id prefer the agent
container sending the info it needs login without hive").
- swarm-queue-client: `agent_login::Login`, the shared wire type, beside
`agent_inference::Effective`. `needs_login: Option<bool>` with
`#[serde(default)]`: a header from an older agent decodes as `None`
(unknown), never as "needs login" or "logged in".
- hive-agent: the header publisher reads the shared LoginState cell.
Every writer of that cell emits a status on the bus afterwards, so a
login change goes out on the next rebuild, not the 60 s refresh.
- swarm-controller: `agent_login::Reported` follows every agent's header
and keeps the latest flag; `GET /api/agents/status` merges it into each
row as `needs_login` (null when unknown).
- swarm-ui: AgentCard shows a warning-tone "login needed" badge when the
row's `needs_login` is true.
Refs #5268
