swarm UI: one request for every agent's linked accounts

GET /api/agents/linked-accounts returns one entry per agent that
/api/agents/status has a row for, as {hive, agent, accounts}, from one
store login. The agents page fetches it once (and again when a link dialog
closes) and hands each table row and the detail panel its agent's slice,
so the page makes no per-agent request. The per-agent route had no caller
left and is removed. The bao grant is unchanged: the same list on each
agent's matrix and forge metadata directories.

Refs #4855

swarm-controller/README.md

@@ -32,8 +32,9 @@ The swarm's control plane. The swarm UI and `swarmctl` are its clients.
   GitHub account for one agent, stored in the swarm secret store
   (`PUT /api/hives/{hive}/agents/{agent}/matrix-accounts/{account}`,
   `.../forge-accounts/{label}`, `.../github-account`); distinct from the agent's own swarm-minted
-  accounts above. `GET .../linked-accounts` lists them, plus the agent's own
-  `main` matrix account, by kind, name and host, never with a credential.
+  accounts above. `GET /api/agents/linked-accounts` lists them for every agent in one
+  response, plus each agent's own `main` matrix account, by kind, name and
+  host, never with a credential.
 - **Config PR status** — each agent's open config-repo PR, cached from forge
   webhooks (`GET /api/config-prs`, `/api/agents/{name}/config-pr`).
 - **Swarm-wide forge objects and webhooks** →