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

docs/agent-lifecycle/persistence.md

@@ -618,10 +618,10 @@ Idempotency is **per-account**: an `avatar-icon-hash` file in each
 account's matrix-sdk `state_dir`. The daemon hashes the PNG bytes and
 skips the upload when unchanged, because every upload mints a fresh
 `mxc://` URI that emits a profile state event in every joined room —
-re-uploading identical bytes is timeline spam. A dashboard-provisioned
-account gets its avatar when the `systemd.paths.hive-matrix-daemon` token
-watcher restarts the daemon (which re-runs the per-account bring-up), so
-no separate avatar trigger is needed. The daemon swallows avatar failures
+re-uploading identical bytes is timeline spam. A swarm-UI-linked
+account gets its avatar when the daemon restarts on a changed account
+listing (exit 75, which re-runs the per-account bring-up), so no separate
+avatar trigger is needed. The daemon swallows avatar failures
 (logged, non-fatal) so they never break account bring-up or sync.
 <!-- vale write-good.Passive = YES -->