agent capabilities live in a swarm KV record, set from swarmctl

An agent's capabilities move from the hive's meta repo
(`meta/capabilities.json`, injected as `HIVE_CAPABILITIES`) into the
`capabilities` field of its key in a new `agent-permissions` KV bucket,
which swarm-controller alone writes.

- swarm-queue-client: `agent_permissions` (bucket, key, `AgentPermissions`,
  direct-get read, the import subject and its wire types).
- swarm-controller: creates the bucket at start; `GET|PUT|DELETE
  /api/agents/{name}/permissions` (deny_unknown_fields and unknown
  capability ⇒ 422, duplicate ⇒ 400, agent must be declared by a hive);
  answers each hive's one-shot import, writing a key only where there is
  none and only for agents that hive's wanted state declares.
- swarmctl: `agent permissions get|set|clear`.
- swarm-nats-auth: a hive reads every key of the bucket and sends its
  import on its own subject; the controller subscribes to the imports and
  may answer them (`responses`).
- hive-c0re: reads each of its agents' keys every 30s into
  /var/lib/hyperhive/swarm-agent-permissions.json and restarts an agent,
  gracefully, when its capabilities change. Unset is the compiled-in
  default (ruth: manage_root_agent; everyone else: none). A failed read
  keeps the stored value. On first boot it seeds that file from
  meta/capabilities.json, sends the grants to the controller, and deletes
  the file once the controller answers.

Deleted: the capabilities.json reader/writer, `HIVE_CAPABILITIES` in the
rendered meta flake, ruth's capability seed, the capabilities half of the
permissions rebuild job (`PermPayload::Capabilities`/`Combined`), the
dashboard's capabilities table and `capabilities_changed` event, and the
now-unused `Capability::description`.

Part of #5170.

hive-c0re/src/swarm_status.rs

@@ -98,7 +98,7 @@ pub fn spawn(
         // reason, and is a third consumer rather than a second connection.
         tokio::spawn(crate::workers::wanted::watch_declarations(
             client.clone(),
-            coord,
+            std::sync::Arc::clone(&coord),
             hive.clone(),
             shutdown.clone(),
         ));
@@ -109,6 +109,14 @@ pub fn spawn(
             shutdown.clone(),
         ));
 
+        // The capabilities the swarm grants, and the import of the meta repo's.
+        tokio::spawn(crate::workers::swarm_permissions::follow(
+            client.clone(),
+            std::sync::Arc::clone(&coord),
+            hive.clone(),
+            shutdown.clone(),
+        ));
+
         let mut health = SweepHealth::new("swarm_status_publish", "warn", FAILURES_BEFORE_BANNER);
         loop {
             match publish(&client, &hive).await {