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-queue-client/Cargo.toml

@@ -50,6 +50,10 @@ schema = ["dep:utoipa"]
 # `agent_limits::read`, for hive-c0re, which reads the key of each agent it
 # runs the same way. Explicit for the same reason as `notices`.
 agent-limits = ["async-nats/jetstream"]
+# `agent_permissions::AgentPermissions` and `read`, for hive-c0re and
+# swarm-controller. Pulls `hive-sh4re` for the `Capability` names, so the
+# crates that only need the bucket name do not link it.
+agent-permissions = ["dep:hive-sh4re", "async-nats/jetstream"]
 
 [dependencies]
 # Bare (no `kv`/`jetstream`) unless a consumer opts into the `kv` feature
@@ -58,6 +62,7 @@ async-nats.workspace = true
 # `Shared`, so every caller refused while a secret re-read runs waits on that
 # one read (`stored_secret`).
 futures-util.workspace = true
+hive-sh4re = { workspace = true, optional = true }
 # `blocking` on top of the workspace default (`form`/`json`/`rustls`) —
 # `mint_token_for_blocking` needs `reqwest::blocking::Client` for a caller
 # with no tokio reactor to `.await` an async request on (an OTLP exporter's