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/job_queue/exec.rs

@@ -810,23 +810,6 @@ async fn run_write_perm_file(
                 .with_context(|| format!("commit tool-groups for {name}"))?;
             coord.emit_tool_groups_snapshot();
         }
-        PermPayload::Capabilities { caps } => {
-            crate::meta::commit_capabilities(name, caps)
-                .await
-                .with_context(|| format!("commit capabilities for {name}"))?;
-            coord.emit_capabilities_snapshot();
-        }
-        PermPayload::Combined { groups, caps } => {
-            crate::meta::commit_perms(name, groups.as_deref(), caps.as_deref())
-                .await
-                .with_context(|| format!("commit perms for {name}"))?;
-            if groups.is_some() {
-                coord.emit_tool_groups_snapshot();
-            }
-            if caps.is_some() {
-                coord.emit_capabilities_snapshot();
-            }
-        }
     }
     Ok(())
 }