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

swarm-controller/src/main.rs

@@ -48,6 +48,7 @@ mod agent_status;
 mod auth;
 mod config_pr;
 mod forge;
+mod forge_account;
 mod issue_report;
 mod matrix_account;
 mod otel_http_client;
@@ -2888,6 +2889,7 @@ fn build_app(state: AppState) -> axum::Router {
         .routes(routes!(get_agents_status))
         .routes(routes!(set_agent_state))
         .routes(routes!(matrix_account::put_matrix_account))
+        .routes(routes!(forge_account::put_forge_account))
         .routes(routes!(get_hive_wanted))
         .routes(routes!(term_stream::stream_agent_term))
         .routes(routes!(agent_state_stream::stream_agent_state))