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-network.nix

@@ -97,8 +97,8 @@ in
       default = false;
       example = true;
       description = ''
-        Flip agent containers from shared host netns to private netns
-        (#14). When true, each agent container gets a dedicated veth
+        Flip agent containers from shared host netns to private netns.
+        When true, each agent container gets a dedicated veth
         pair attached to `bridgeName` and a deterministic IP from
         the bridge subnet. The bridge (already up when `enable = true`)
         becomes the sole routed path between the host and agent