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/workers/mod.rs

@@ -1,7 +1,7 @@
 //! Background tasks and periodic sweeps: crash/login watcher, boot-time
 //! auto-update reconcile, the agent-sockets.json writer loop, the MCP socket listener
 //! reconcile loop, knowledge-repo sync, the swarm wanted-state pull, and the
-//! swarm resource-cap read.
+//! swarm resource-cap and permissions reads.
 //! Each submodule is re-exported
 //! at the crate root, so `crate::crash_watch::…` etc. keep working unchanged.
 
@@ -10,5 +10,7 @@ pub mod auto_update;
 pub mod crash_watch;
 pub mod knowledge;
 pub mod mcp_sockets;
+pub mod permissions_import;
 pub mod swarm_limits;
+pub mod swarm_permissions;
 pub mod wanted;