swarm UI: an agent that needs a Claude login shows it on its card

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

swarm-queue-client/src/lib.rs

@@ -221,6 +221,10 @@ pub mod agent_inference;
 /// writing needs `kv` and reading needs `agent-limits`.
 pub mod agent_limits;
 
+/// The login flag in an agent's turn-state header. No bucket: it travels
+/// on [`agent_state_subject`] only.
+pub mod agent_login;
+
 #[cfg(feature = "kv")]
 mod kv;