extra-forges: fully dashboard-provisioned, no host config

Per mara's feedback on PR #2407 ("better: you can also provide url in
dashboard, same as with matrix, no host config"), drops
services.hyperhive.extraForges and the admin-API mint/revoke flow
entirely. The operator now creates a token on the external forge
themselves and pastes a label + base URL + access token into the
dashboard's FORGES tab, the same shape as the GitHub PAT flow plus the
base-URL field from the matrix extra-account flow. hive-c0re only ever
writes/deletes two local files per account (forge-<label>-token,
forge-<label>.json sidecar for the URL) via hive-priv — no remote
account creation, no admin token, no revoke-on-the-remote-side, no nix
config to enumerate.

- nix/host-modules/hive-forge/default.nix: removed the extraForges
  option, its label-format assertion, and the HYPERHIVE_EXTRA_FORGES
  env forwarding.
- hive-c0re/src/forge/extra.rs: deleted (REST admin-API provisioning,
  no longer needed).
- hive-c0re/src/dashboard/extra_forges.rs: GET /api/extra-forges?
  agent= lists an agent's stored forges by scanning its state dir
  (mirrors matrix_accounts.rs's filename-scan listing), POST
  /api/extra-forge-account (agent/label/base_url/token/
  action=add|remove) stores or removes an account.
- hive-sh4re/priv_proto.rs + hive-priv/main.rs: new
  WriteAgentExtraForgeAccount/DeleteAgentExtraForgeAccount priv
  requests (adds base_url, writes/deletes a JSON sidecar alongside the
  token).
- hive-c0re/src/priv_client.rs: matching wrapper functions.
- frontend/packages/dashboard/src/credentials.{html,js}: FORGES tab is
  a per-agent list + add-account paste form (label/base_url/token), no
  grant/revoke-from-catalog UI.
- docs/web-ui/dashboard.md: FORGES tab section rewritten.

Supersedes the design in PR #2407 (already approved+green on the old
admin-API model) — opening as a fresh PR against the same issues
rather than force-pushing over the approved one.

hive-c0re/src/priv_client.rs

@@ -300,6 +300,37 @@ pub async fn write_agent_github_token(agent_name: &str, token: &str) -> Result<(
     .await?)
 }
 
+/// Write a per-agent account for a dashboard-declared external forge —
+/// label + base URL + token — to `<state>/forge-<label>-token` +
+/// `<state>/forge-<label>.json` via hive-priv. Entirely dashboard-
+/// provisioned, no host-side nix config; `label` is validated root-side as
+/// a plain identifier before it reaches the filename.
+pub async fn write_agent_extra_forge_account(
+    agent_name: &str,
+    label: &str,
+    base_url: &str,
+    token: &str,
+) -> Result<()> {
+    ok(call(&PrivRequest::WriteAgentExtraForgeAccount {
+        agent_name: agent_name.to_owned(),
+        label: label.to_owned(),
+        base_url: base_url.to_owned(),
+        token: token.to_owned(),
+    })
+    .await?)
+}
+
+/// Remove a previously-added extra-forge account — the counterpart of
+/// [`write_agent_extra_forge_account`]. Idempotent: missing files are not
+/// an error.
+pub async fn delete_agent_extra_forge_account(agent_name: &str, label: &str) -> Result<()> {
+    ok(call(&PrivRequest::DeleteAgentExtraForgeAccount {
+        agent_name: agent_name.to_owned(),
+        label: label.to_owned(),
+    })
+    .await?)
+}
+
 /// Restart `hive-matrix-daemon.service` inside an agent container via
 /// `systemctl --machine=h-<agent_name> restart hive-matrix-daemon.service`.
 /// Non-fatal: callers should handle errors gracefully — if the container is