forge: external forge accounts live in swarm bao; the agent fetches them itself

An operator now links an agent's external forge account (label, base URL,
token) in the swarm UI. swarm-controller stores it at
swarm/agents/<agent>/forge/<label>. There is no index: the store's
listing of the agent's forge/ directory is the set of accounts.

In the agent, hive-agent-forge-accounts (oneshot + 2-minute timer, as
the agent user, under its own store certificate) lists
swarm/agents/<agent>/forge/ with the `list` #4866 grants an agent on its
own metadata subtree, reads each account, and writes
<state>/forge-<label>-token and forge-<label>.json in the names and shape
hive-forge -f already reads. An empty listing (a 404, which `bao kv list
-format=json` answers with `{}` and an empty stderr) is zero accounts; a
denial or an unreachable store fails the unit. It never deletes: files
for labels not listed, including ones the hive wrote, stay as they are.

Removed: the dashboard FORGES tab (credentials.js/html section and its
CSS), hive-c0re's extra_forges.rs and its routes, priv_client's
extra-forge calls, and hive-priv's WriteAgentExtraForgeAccount /
DeleteAgentExtraForgeAccount with their helpers. The GITHUB tab and
WriteAgentGithubToken stay.

Also: persistence.md's matrix avatar note names the exit-75 restart on a
changed account listing, not the dashboard, as what brings a linked
account up.

Refs #4348

frontend/packages/dashboard/src/credentials.css

@@ -81,43 +81,6 @@ body.cred-shell {
   border-color: var(--purple);
 }
 
-.ma-accounts {
-  list-style: none;
-  padding: 0;
-  margin: 0;
-}
-.ma-account {
-  display: flex;
-  align-items: center;
-  gap: 0.6rem;
-  padding: 0.45rem 0.2rem;
-  border-bottom: 1px solid var(--border);
-}
-.ma-dot {
-  width: 0.6rem;
-  height: 0.6rem;
-  border-radius: 50%;
-  flex: none;
-}
-.ma-dot.ok {
-  background: var(--green);
-}
-.ma-name {
-  font-weight: 600;
-  color: var(--fg);
-}
-.ma-hs {
-  color: var(--muted);
-  font-size: 0.85rem;
-}
-.ma-status {
-  margin-left: auto;
-  font-size: 0.8rem;
-}
-.ma-status.ok {
-  color: var(--green);
-}
-
 .ma-result {
   margin-top: 0.7rem;
   font-size: 0.9rem;