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.

frontend/packages/dashboard/src/system-sections.css

@@ -201,9 +201,9 @@
    sub-step label` — the rule has had no JS referents since then). */
 
 /* ─── K3PT ST4T3 › stale permission entries sub-section ────────────────────
-   Agents with explicit capability / tool-group entries in the JSON but no
-   live container (renamed or manually-deleted agents). Lazy-loaded on tab
-   activation, refreshed on capabilities_changed / tool_groups_changed. */
+   Agents with explicit tool-group entries in the JSON but no live
+   container (renamed or manually-deleted agents). Lazy-loaded on tab
+   activation, refreshed on tool_groups_changed. */
 .tombstones-stale-heading {
   margin: 1.2em 0 0.3em;
   color: var(--muted);