chore(nix): replace tracker tags with prose in nix comments

Part of the tracker-tag cleanup: the hive convention is prose, not
issue-tracker tags, in code. Reword the 21 tags in the nix tree
(flake.nix + the hive-c0re/ci/gateway/network modules) to describe
the thing they pointed at, preserving the context without the tag.

Comment-only — no eval or logic change. Validated with nix fmt
(no reformatting) and nix flake check --no-build (all checks
evaluate clean); the full build check was skipped locally because
the shared remote builder is degraded, so CI will exercise the
build derivations once the runner recovers.

nix/modules/hive-c0re.nix

@@ -458,7 +458,7 @@ in
         Pre-fetch the per-container system closures (agent-base +
         manager toplevels) into the host's /nix/store as part of this
         host's NixOS build, instead of letting the first agent spawn
-        do all the work. Closes #97.
+        do all the work.
 
         Enabling this adds roughly the full nixpkgs runtime closure +
         claude-code + the harness binary to your system closure size
@@ -612,8 +612,8 @@ in
       pkgs.git
     ];
 
-    # Pull the per-container toplevels into the host system closure
-    # (#97). `system.extraDependencies` adds paths to the system build
+    # Pull the per-container toplevels into the host system closure.
+    # `system.extraDependencies` adds paths to the system build
     # without referencing them at runtime — nixos-rebuild fetches /
     # builds them, they end up in /nix/store, and the first
     # nixos-container update + start for an agent has nothing left to