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/templates.rs

@@ -403,8 +403,8 @@ pub fn destroy(builder: &JobBuilder, agent: &str, purge: bool) {
         .after_ok(last);
 }
 
-/// Perm change: commit the JSON file(s), then the rebuild subgraph so
-/// the updated `HIVE_TOOL_GROUPS` / `HIVE_CAPABILITIES` env var takes
+/// Perm change: commit the JSON file, then the rebuild subgraph so
+/// the updated `HIVE_TOOL_GROUPS` env var takes
 /// effect in the container. Group-roots are `WritePermFile` plus the rebuild
 /// subgraph's `MetaSync` / `Prebuild` / `Reconcile`, so the `EmitRebuilt` tail
 /// edges all four.