hive-gateway: stop restating what gateway.md already says

Two comment blocks explained things `docs/networking/gateway.md` already
explains, one of them directly above a line pointing at that very doc. A
pointer under a duplicate is not a pointer; it is a duplicate with a
citation.

Measured, not eyeballed: 6-gram intersection between this file's comments
and that doc, 73 -> 31. The residual is mostly the one-line summaries kept
at the call sites, which necessarily share phrasing with the doc they
summarise — that is the floor of the metric, not remaining duplication.

  * the self-signed-cert import unit: kept the 🚨 that stops someone
    collapsing it into "point nginx at the CA dir", both jobs at one line
    each, and the blast radius (whole gateway, not one vhost). Dropped the
    `BIO_new_file()` mechanism and the production-outage narrative — the
    doc carries both, and the second is history.
  * the nginx-reload note: rewritten to say the one thing this file needs
    to say, that nothing here reloads nginx and hive-c0re drives it from
    the host through hive-priv. The per-unit-state dispatch table is the
    doc's, and only the doc has it in full.

Both pointers use the `docs/x.md::Section` form and name headings verified
to exist; the first replaces a parenthetical `("Self-signed TLS")` spelling
that no check can resolve.

Comments only; no behaviour change. Refs #3901.

nix/host-modules/hive-gateway/default.nix

@@ -289,31 +289,20 @@ in
 
     # Import the hive-CA leaf into nginx's state dir before nginx starts.
     #
-    # 🚨 DO NOT "simplify" this into pointing nginx at the CA dir. It
-    # does TWO jobs:
+    # 🚨 DO NOT "simplify" this into pointing nginx at the CA dir. It does
+    # TWO jobs: it re-modes the leaf to the 0640 root:nginx copy below (a
+    # 0600 key fails nginx's pre-start config test and blocks the unit),
+    # and it guarantees every cert path the config names exists — which is
+    # what the swarm-services fallback at the bottom of the script is for.
+    # A leaf that never issues therefore takes the WHOLE gateway down, not
+    # one vhost. Both mechanisms, and what it cost the time this was
+    # collapsed: `docs/networking/gateway.md::Self-signed TLS (default)`.
     #
-    # (1) It re-modes the leaf. `hive-tls-ca` writes the key 0600
-    #     root:root; nginx's pre-start `nginx -t` runs as the nginx
-    #     *user*, so a 0600 key fails the config test with
-    #     `BIO_new_file() … Permission denied` and blocks the unit —
-    #     hence the 0640 root:nginx copy below.
-    # (2) It guarantees that **every cert path the nginx config names
-    #     exists** — which is what the swarm-services fallback at the
-    #     bottom of the script is for. nginx refuses to load a config
-    #     naming a missing cert file, so a leaf that never issues takes
-    #     the whole gateway down rather than one vhost; that has already
-    #     happened once and it took the forge, dashboard and matrix with
-    #     it. Removing this unit re-creates it exactly.
-    #
-    # nginx
-    # `Requires=` this via `requiredBy`, so it refuses to start until
-    # the copy succeeds. ALWAYS runs (no ConditionPathExists) and is
-    # idempotent — necessary to reconcile broken state from prior
-    # failed boots (a 0700 dir from a stale UMask, a truncated copy
-    # from an interrupted oneshot, etc.). The leaf covers the bare
-    # hive domain plus `forge.`, `matrix.` and `*.${hyperhiveDomain}`
-    # so all sub-domains validate under the same cert + the hive CA.
-    # See `docs/networking/gateway.md` ("Self-signed TLS").
+    # ALWAYS runs (no ConditionPathExists) and is idempotent, because it
+    # has to reconcile broken state from earlier failed boots — a 0700 dir
+    # from a stale UMask, a truncated copy from an interrupted oneshot.
+    # nginx `Requires=` it via `requiredBy`, so nginx refuses to start
+    # until the copy succeeds.
     systemd.services.hive-gateway-self-signed-cert = lib.mkIf useSelfSigned {
       description = "Import host-generated TLS leaf for hive-gateway";
       wantedBy = [ "multi-user.target" ];
@@ -383,14 +372,11 @@ in
       '';
     };
 
-    # nginx reload is triggered from the HOST side by hive-c0re
-    # after each agents.conf write, through hive-priv (c0re is
-    # unprivileged and cannot act on a system unit).
-    #
-    # It stays an explicit trigger rather than a systemd path unit
-    # watching the file: the write and the reload belong in one causal
-    # chain c0re can retry and report on (see RELOAD_PENDING), not two
-    # independent units racing on an inotify event.
+    # Nothing here reloads nginx: hive-c0re drives that from the host side
+    # through hive-priv after each agents.conf write, deliberately as an
+    # explicit trigger rather than a path unit. Why, and what hive-priv
+    # dispatches per unit state:
+    # `docs/networking/gateway.md::Per-agent unix-socket upstream`.
 
     services.nginx = {
       enable = true;