remove the forge links from the hive dashboard and the agent page

Each agent row on the hive dashboard, and the agent page's link menu, linked
to the agent's forge profile and its config repo. Both are gone, with what
only they used:

- hive-agent no longer builds the two links, the `Forge` link kind, or the
  `forge_public_url` field of its state, and no longer reads
  `HIVE_FORGE_PUBLIC_URL`. The agent page's link menu and the dashboard row's
  icon strip (which skips a forge link an agent not yet rebuilt still sends)
  drop their forge handling.
- hive-c0re's dashboard state no longer carries `forge_public_url`.
- `HIVE_FORGE_PUBLIC_URL` is no longer forwarded to agents through the meta
  flake (the forwarded-variable list, its test fixture, the controller's
  settings and the VM test), so the next meta flake render differs by that
  one line. hive-c0re still reads it for the forge URL a hive publishes, so
  `swarm.forge.publicUrl` stays, with its description corrected.
- The docs stop describing the links.

docs/4-implementation/networking/gateway.md

@@ -431,9 +431,7 @@ when `StateSnapshot.gateway_enabled` is true, and direct
 every hive, so the direct shape is a fallback for the variable being unset.
 Three render sites follow it: the agent-name link, the favicon fetch
 (`<url>/icon`), and the nav-strip `container`-kind links from
-`DashboardState.links` (`GET /api/dashboard-state`). Forge links come from
-`swarm.forge.publicUrl`; with it unset the dashboard hides them rather than
-guess. See `docs/2-handbook/hives/dashboard.md::Container row` for the frontend side.
+`DashboardState.links` (`GET /api/dashboard-state`). See `docs/2-handbook/hives/dashboard.md::Container row` for the frontend side.
 
 ### Dialing another vhost by name (`verifiedProxyTo`)