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/dashboard.css

@@ -566,57 +566,9 @@ footer .banner-thin {
   margin-bottom: 0.8em;
 }
 
-/* ─── capabilities (permissions) table ─────────────────────────────
-   Agents × capabilities matrix in the P3RM1SS10NS tab. Same layout
-   as the tool-groups table below. Horizontally scrollable on narrow
-   viewports. */
-.cap-table-wrap {
-  overflow-x: auto;
-  margin-top: 0.5em;
-}
-.cap-table {
-  border-collapse: collapse;
-  font-size: 0.82em;
-  min-width: 100%;
-}
-.cap-table th,
-.cap-table td {
-  padding: 0.35em 0.6em;
-  border: 1px solid var(--border);
-  text-align: center;
-  vertical-align: middle;
-}
-.cap-table thead th {
-  background: var(--bg-elev);
-  color: var(--muted);
-  letter-spacing: 0.05em;
-  white-space: nowrap;
-}
-.cap-agent-col {
-  text-align: left !important;
-  min-width: 8em;
-}
-.cap-col {
-  min-width: 5em;
-}
-.cap-agent-name {
-  color: var(--fg);
-  font-weight: 600;
-}
-.cap-cb {
-  cursor: pointer;
-  width: 1em;
-  height: 1em;
-  accent-color: var(--purple);
-}
-.cap-row:hover td {
-  background: var(--bg-elev);
-}
-
 /* ─── tool-groups (permissions) table ──────────────────────────────
-   Agents × tool-groups matrix in the P3RM1SS10NS tab. Same layout
-   as the capabilities table above. Horizontally scrollable on narrow
-   viewports. */
+   Agents × tool-groups matrix in the P3RM1SS10NS tab. Horizontally
+   scrollable on narrow viewports. */
 .tg-table-wrap {
   overflow-x: auto;
   margin-top: 0.5em;