fix(#2860): make hyperhive.forge.url required, drop the loopback default

The default `http://localhost:3000` was a guess, and a guess that
evaluates is worse than one that doesn't: the forge may run on a
different host from the agents, and inside an agent's network namespace
`localhost` reaches the agent itself, not the forge. hive-c0re renders
this option into every agent's config from the host's `HIVE_FORGE_URL`,
which `hive-c0re.nix` sets unconditionally, so nothing legitimate was
relying on the default to be there.

Also tightens the URL assertion, which accepted `""`. That escape hatch
only existed because the default made the empty case unreachable; with
the default gone, `""` is exactly what a caller supplies when they have
nothing, so it now fails naming the option instead of silently building
a config that talks to nowhere.

Refs #2860

nix/agent-modules/forge.nix

@@ -21,7 +21,6 @@ in
 {
   options.hyperhive.forge.url = lib.mkOption {
     type = lib.types.str;
-    default = "http://localhost:3000";
     example = "http://forge.internal:3000";
     description = ''
       Base URL of the hyperhive-managed Forgejo. Used at container
@@ -31,16 +30,27 @@ in
       shell out to `tea` without an extra auth dance. No-op when the
       forge-token file is missing (i.e. hive-forge isn't running on
       the host).
+
+      **Required, deliberately undefaulted.** hive-c0re renders it into
+      every agent's config from the host's `HIVE_FORGE_URL`, which
+      `hive-c0re.nix` sets unconditionally --- the forge is mandatory.
+      A loopback default would be a guess: the forge may run on a
+      different host from the agents, and inside an agent's network
+      namespace `localhost` reaches the agent, not the forge. An
+      unevaluatable config is better than one that builds and then
+      talks to the wrong machine.
     '';
   };
 
   config = {
     assertions = [
-      # hyperhive.forge.url must look like an HTTP URL when non-default.
+      # The empty string is the one value the type permits that cannot
+      # be a URL, and it is what a caller supplies when they have
+      # nothing --- exactly the case the removed loopback default used
+      # to paper over. Reject it here so the failure names the option.
       {
         assertion =
-          config.hyperhive.forge.url == ""
+          lib.hasPrefix "http://" config.hyperhive.forge.url
-          || lib.hasPrefix "http://" config.hyperhive.forge.url
           || lib.hasPrefix "https://" config.hyperhive.forge.url;
         message = "hyperhive.forge.url must be an http:// or https:// URL (got: \"${config.hyperhive.forge.url}\")";
       }