swarm: show subagent terminals in the swarm UI

An agent's subagent daemon publishes each subagent's output as terminal
rows on `$SWARM.term.<agent>.sub.<subagent>`, as the agent, into a
per-agent stream it creates itself; swarm-controller lists an agent's
subagents from that stream's subjects and relays one subagent's rows as
SSE; the swarm UI lists them under the agent's terminal preview and
reuses AgentTermPreview, full-screen tab included, with no input.

- swarm-nats.nix: the agent token may also publish
  `$SWARM.term.{agent}.sub.>` and `$JS.API.STREAM.CREATE|INFO` on
  `term-sub-{agent}`, and nothing else of JetStream. A module-eval arm
  pins the agent-token grant as an exact list.
- mcp.nix: hive-subagent-daemon loads the agent's store identity
  (`hive-agent-bao-cert/-key/-server-ca`, the ones hive-agent loads)
  whenever the agent has a store, not only on the opencode preset. The
  agent's own queue secret lives in the store, so this is the credential
  the harness connects with.
- hive-subagent-mcp: `swarm_term` reads the agent's queue secret under
  that identity, connects with the agent token, opens or creates
  `term-sub-<agent>` (max_age 24h), and publishes classified rows from
  the sink every subagent line already passes through. The sink only
  queues (bounded, drop-and-count); a missing store, refused credential,
  failed stream create or failed publish is a log line.
- The stream-json classifier (`stream_enrich`) and the `TermMsg` row
  types plus `fit` move from the hive-agent binary into hive-sh4re, so
  the subagent daemon publishes the rows AgentTermPreview already
  renders. hive-agent keeps its LiveEvent classifier on top.
- swarm-controller: `GET /api/agents/{name}/subagents` and
  `GET /api/agents/{name}/subagents/{subagent}/term/stream`.
- docs/swarm: what the UI shows and what the queue carries.

Closes #4827

nix/agent-modules/mcp.nix

@@ -31,11 +31,14 @@ let
   # (several agents that rarely compile at the same time) working.
   subagentMemoryHigh = containerMemoryMaxBytes * 2 / 3;
   # Set on the harness only for an ACP agent on the opencode preset
-  # (./agent-service.nix). The subagent daemon then reads that key from the
-  # store as this agent, so it is handed the same store identity
-  # ./queue-identity.nix hands the harness.
+  # (./agent-service.nix).
   acpApiKeyEnv = config.systemd.services.hive-agent.environment.HIVE_ACP_API_KEY_ENV or null;
-  subagentReadsStore = acpApiKeyEnv != null && config.services.hyperhive.agent.bao.addr != null;
+  # The subagent daemon reads this agent's own queue credential from the
+  # store, to publish its subagents' terminals as the agent, and on the
+  # opencode preset the provider key too. Either way it is handed the same
+  # store identity ./queue-identity.nix hands the harness, under the same
+  # switch.
+  subagentReadsStore = config.services.hyperhive.agent.bao.addr != null;
 in
 {
   options.services.hyperhive.agent.allowedRecipients = lib.mkOption {