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

docs/swarm/ui.md

@@ -22,7 +22,9 @@ domain, covers host-level detail for one hive.
 
 Everything it shows comes from [`swarm-controller`](../../swarm-controller/README.md).
 An agent created here or with `swarmctl agent create` starts `paused`; set it
-`up` from its card.
+`up` from its card. A card shows **login needed** when the agent's own
+turn-state header says its claude has no session. Log it in from the agent's
+own web UI.
 
 ### Agent and subagent terminals