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.

swarm-controller/src/main.rs

@@ -47,6 +47,7 @@ mod agent_identity;
 mod agent_input;
 mod agent_limits;
 mod agent_login;
+mod agent_permissions;
 mod agent_renewal;
 mod agent_state_stream;
 mod agent_status;
@@ -2877,11 +2878,22 @@ fn spawn_subagent_streams(status: Option<&Arc<status::StatusReader>>, hives: &[H
 }
 
 /// The per-agent background passes: subagent and terminal streams, and
-/// schedule keys.
+/// schedule keys, and the permissions bucket and its import.
 /// Lifted out of `main` for `clippy::too_many_lines`.
 fn spawn_per_agent_upkeep(status: Option<&Arc<status::StatusReader>>, hives: &[HiveEntry]) {
     spawn_subagent_streams(status, hives);
     spawn_schedule_reconcile(status, hives);
+    spawn_agent_permissions(status, hives);
+}
+
+/// Create the agent-permissions bucket and answer hives' imports when a swarm
+/// queue is wired up. Same shape as [`spawn_subagent_streams`].
+fn spawn_agent_permissions(status: Option<&Arc<status::StatusReader>>, hives: &[HiveEntry]) {
+    let (Some(status), Some(wanted)) = (status, wanted_writer(status)) else {
+        return;
+    };
+    let hives = hives.iter().map(|h| h.name.clone()).collect();
+    agent_permissions::spawn(status.queue_client(), wanted, hives);
 }
 
 /// Start rewriting every agent's schedule key from the definitions when a
@@ -3493,6 +3505,11 @@ fn build_app(state: AppState) -> axum::Router {
             agent_limits::put_agent_limits,
             agent_limits::delete_agent_limits
         ))
+        .routes(routes!(
+            agent_permissions::get_agent_permissions,
+            agent_permissions::put_agent_permissions,
+            agent_permissions::delete_agent_permissions
+        ))
         .routes(routes!(get_config_prs))
         .routes(routes!(create_agent))
         .routes(routes!(mint_agent_forge_token))