swarm UI: show the accounts linked to each agent

GET /api/hives/{hive}/agents/{agent}/linked-accounts returns one row per
account linked to the agent, as kind, name and host: each matrix account
under swarm/agents/<agent>/matrix (with its homeserver, and the agent's own
`main` marked reserved), each forge label under swarm/agents/<agent>/forge
(with its url), and github when swarm/agents/<agent>/github-token exists
(host github.com, which is not stored). No credential field is in the
response type.

Listing those two directories needs a new controller grant: `list` on
secret/metadata/swarm/agents/+/matrix and .../+/forge only, pinned in
bao-grants.nix as the only metadata stanzas under agents/ beside the queue
revocation. Checked against a dev OpenBao 2.6.3: the grant lists those two
directories and is refused on agents/, agents/<agent>/, and a leaf.

The swarm UI agent detail panel shows all rows under "accounts"; the table
view's matrix column shows the matrix rows. The link badges stay.

Refs #4855

nix/host-modules/swarm-bao.nix

@@ -400,6 +400,14 @@ let
       capabilities = ["delete"]
     }
 
+    path "${credentialMountPath}/metadata/swarm/agents/+/matrix" {
+      capabilities = ["list"]
+    }
+
+    path "${credentialMountPath}/metadata/swarm/agents/+/forge" {
+      capabilities = ["list"]
+    }
+
     path "${credentialMountPath}/data/swarm/hives/+/matrix/sender-token" {
       capabilities = ["create", "read", "update"]
     }