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/forge/users.rs

@@ -31,7 +31,10 @@ use crate::paths::FORGE_CORE_TOKEN as CORE_TOKEN_PATH;
 // build.
 /// Per-agent token scopes (broad-but-not-admin). See
 /// `docs/forge.md::Token scopes` for the per-scope rationale.
-const TOKEN_SCOPES: &str = "read:user,write:user,read:notification,write:notification,write:repository,write:issue,write:organization,write:misc";
+/// `pub(super)` — also reused by `extra.rs`'s external-forge
+/// provisioning so a granted agent gets the same scope set on an
+/// extra forge as on the internal one.
+pub(super) const TOKEN_SCOPES: &str = "read:user,write:user,read:notification,write:notification,write:repository,write:issue,write:organization,write:misc";
 
 /// Bootstrap `core` token scopes — adds `read:admin,write:admin` on
 /// top of `TOKEN_SCOPES` so the host daemon can drive