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.

docs/agent-lifecycle/persistence.md

@@ -391,10 +391,6 @@ Contents:
   (`{ "alice": ["messaging", "inbox", "execution"] }`). Written by
   `tool_groups::set_groups`; injected as `HIVE_TOOL_GROUPS` env
   var into each agent's container.
-- `capabilities.json` — per-agent capability grants
-  (`{ "ruth": ["manage_root_agent"] }`). Written by
-  `capabilities::set_caps`; injected as `HIVE_CAPABILITIES` env
-  var. Absent agents have no extra capabilities.
 - `swarm-resource-limits.json` — the caps the swarm sets for this
   hive's agents
   (`{ "sock": { "cpu_quota": "400%", "memory_max": "8G" } }`).
@@ -418,6 +414,13 @@ Contents:
 
 The root agent has the meta dir RO-mounted at `/meta/`.
 
+Outside the meta repo, `/var/lib/hyperhive/swarm-agent-permissions.json`
+holds the capabilities the swarm grants this hive's agents, as
+`workers::swarm_permissions` last read them from the swarm's
+`agent-permissions` bucket (every 30 seconds): a copy, not the
+record, which `swarmctl agent permissions` sets. `set_nspawn_flags`
+reads it at every container start.
+
 hive-c0re renders containers onto `meta#<n>` at creation.
 
 ## Destroy vs purge