hive-c0re: stop naming nix implementation files in env-var prose

Six diagnostics and eight comments explained where an environment
variable comes from by naming the module file that sets it. The reader
of a panic is an operator, who does not edit that file; the reader of
the comment is already in the code. Neither can act on a filename, and
a filename in prose is checked by nothing.

Each one now names what the reader can act on: the option where there
is one (services.hyperhive.network.bridgeName, .bridgeIp,
.bridgePrefixLength, c0re.assets, c0re.orgAvatarPng), otherwise the
unit the process is expected to run under. Two of the six diagnostics
had a real option available; the other four have none, so they name
the unit rather than an invented path.

Four of them pointed at hive-c0re.nix, which is a directory. Two of
those spelled it nix/modules/hive-c0re.nix — a directory that does not
exist either, and one of the two was wrapped across a line break, so
no grep for the whole path could have found it. That is the argument
for the change: prose naming a file rots silently because nothing
compiles it.

Scope here is the four files around those diagnostics. The same
pattern still matches elsewhere in the tree; that sweep follows.

hive-c0re/src/forge/mod.rs

@@ -39,7 +39,8 @@ use users::{
 const FORGE_CONTAINER: &str = "hive-forge";
 
 /// Base HTTP URL for the local Forgejo instance, from `HIVE_FORGE_URL`
-/// (set unconditionally by `hive-c0re.nix` to `http://<forge.domain>`).
+/// (set unconditionally by the hyperhive NixOS module on the `hive-c0re`
+/// unit, to `http://<forge.domain>`).
 ///
 /// # Panics
 ///
@@ -54,8 +55,9 @@ pub(crate) fn forge_http_base() -> &'static str {
     static BASE: OnceLock<String> = OnceLock::new();
     BASE.get_or_init(|| {
         std::env::var("HIVE_FORGE_URL").expect(
-            "HIVE_FORGE_URL is unset — hive-c0re.nix sets it unconditionally, \
-             so this process was started outside the NixOS module",
+            "HIVE_FORGE_URL is unset — the hyperhive NixOS module sets it \
+             unconditionally on the hive-c0re unit, so this process was \
+             started outside that unit",
         )
     })
 }