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-gateway.nix

@@ -973,8 +973,8 @@ in
               # Hive authoritative records — answer queries for the
               # hive domain + its sub-domains with the bridge IP
               # (where nginx is reachable from container netns once
-              # #14 lands; today it's the host loopback alias and
-              # works in either shape).
+              # per-agent netns isolation lands; today it's the host
+              # loopback alias and works in either shape).
               #
               # The forge / matrix entries are redundant in the
               # common case where `forge.domain` /