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.

hive-c0re/src/dashboard/state_snapshot.rs

@@ -66,15 +66,6 @@ pub(super) struct StateSnapshot {
     /// `http://<hostname>:<port>/` TCP links) is retained as a defensive
     /// fallback for the env being unset. See `docs/4-implementation/networking/gateway.md::Vhost map`.
     gateway_enabled: bool,
-    /// Public URL of the forge (e.g.
-    /// `"https://forge.pr1ma.darkest.space"`). Sourced from the
-    /// `HIVE_FORGE_PUBLIC_URL` env var, which the c0re NixOS module
-    /// sets from `services.hyperhive.swarm.forge.publicUrl` (defaults to
-    /// the gateway vhost URL). `None` when absent — the frontend **hides**
-    /// forge links rather than guessing `http://<hostname>:3000`,
-    /// which is only right by accident on deployments that aren't
-    /// plain localhost.
-    forge_public_url: Option<String>,
     /// Browser URL of the swarm UI (the swarm apex domain). Sourced from
     /// `HIVE_SWARM_PUBLIC_URL`, which the c0re NixOS module sets from
     /// `services.hyperhive.swarm.domain`. `None` when the domain is unset —
@@ -197,9 +188,6 @@ pub(super) async fn api_state(
             let s = v.to_string_lossy().to_ascii_lowercase();
             matches!(s.as_str(), "1" | "true" | "yes")
         }),
-        forge_public_url: std::env::var("HIVE_FORGE_PUBLIC_URL")
-            .ok()
-            .filter(|s| !s.is_empty()),
         swarm_public_url: std::env::var("HIVE_SWARM_PUBLIC_URL")
             .ok()
             .filter(|s| !s.is_empty()),