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-controller/Cargo.toml

@@ -66,6 +66,9 @@ sha2.workspace = true
 # `forge/pull_diff.rs` diffs a PR's changed files itself, at a context width
 # the forge's own `.diff` does not offer.
 similar.workspace = true
+# The `Capability` names `/api/agents/{name}/permissions` accepts, the same
+# enum the hive enforces.
+hive-sh4re.workspace = true
 # Validates `POST /api/agents`' `name` before it becomes `agent`/`repo`
 # everywhere downstream — see `create_agent`'s doc comment for why this is
 # defense-in-depth, not the only gate (per an argus review finding).
@@ -102,6 +105,7 @@ swarm-authelia-bridge-sock.workspace = true
 # carries.
 swarm-queue-client = { workspace = true, features = [
   "kv",
+  "agent-permissions",
   "subagent-term",
   "agent-term",
   "schema",