swarm UI: send a message to an agent, without its hive

The agent view in the swarm UI gets a message box. The text reaches the
agent's harness straight from swarm-controller over the swarm queue and
becomes an inbox turn from the operator's authelia username. hive-c0re is
not in the path.

- gateway: `/api/` forwards authelia's `Remote-User`, replacing any the
  client sent.
- swarm-controller: `POST /api/agents/{name}/input` takes that header as
  the sender (401 without it) and sends `{from, body}` as a NATS request
  on `$SWARM.input.<agent>`. No subscriber is a 503.
- swarm-nats-auth: the controller is granted publish on `$SWARM.input.*`;
  no hive or agent grant names the family, so the harness can trust
  `from`.
- hive-agent: a task subscribes to the agent's own input subject and
  pushes each message into the serve loop's existing reminder channel,
  renamed `local_msg_*` since it now carries both.

The harness does not reply to the request: no agent grant covers a reply
subject. The controller therefore answers 202 once the request times out
(2s) and 503 on the server's no-responders status.

Closes #4828

frontend/packages/swarm-ui/src/pages/agents/AgentTermPreview.tsx

@@ -1,7 +1,7 @@
 // <AgentTermPreview> — the MVP mara's own ruling asked for: not a separate
-// page, a small read-only preview embedded below `AgentsPage`'s detail
-// panel fields. No input — sending input back to the agent is explicitly
-// follow-up scope, not this issue's. Consumes swarm-controller's
+// page, a small preview embedded below `AgentsPage`'s detail panel fields, with
+// an `AgentInputForm` under an agent's terminal (a subagent's has none: it
+// takes no input of its own). Consumes swarm-controller's
 // `GET /api/agents/{name}/term/stream`, or for a subagent
 // `GET /api/agents/{name}/subagents/{subagent}/term/stream`, via
 // `useSwarmTermStream`, rendering through the same `@hive/shared` `Row`
@@ -49,6 +49,7 @@ import {
   useSwarmAgentStateStream,
   type TokenUsage,
 } from "./useSwarmAgentStateStream.js";
+import { AgentInputForm } from "./AgentInputForm.js";
 import "./AgentTermPreview.css";
 
 const NEAR_BOTTOM_PX = 48;
@@ -256,6 +257,7 @@ export function AgentTermPreview({
           )}
         </div>
       </div>
+      {subagent === undefined && <AgentInputForm agentName={agentName} />}
     </div>
   );
 }