docs: point matrix GUI comments at the swarm UI, drop the M4TR1X tab

The previous commit removed dashboard.md's M4TR1X section. These
comments and the `deploy.matrix.gui.enable` option description still
described a hive-dashboard M4TR1X tab or cited that section. They now
state what the option does: it serves the client on the gateway vhost
and adds the swarm UI's Matrix quick link (docs/swarm/ui.md::Quick links).

Refs #3902

frontend/packages/dashboard/src/dashboard.html

@@ -91,11 +91,11 @@
         <!-- (Peer hives are no longer a tab — they render as a headline
            under SW4RM. See tab-pane-swarm.) -->
 
-        <!-- FL0W, L0GS, and M4TR1X are separate pages reachable from the
-           H0M3 hub (served at /), not from the dashboard tab strip — the
-           strip holds only real in-page tabs now. Contextual deep-links
-           into the log viewer (an agent's logs, a build entry's log)
-           still live inside the relevant dashboard content. -->
+        <!-- FL0W, L0GS, and the other standalone pages are reachable from
+           the H0M3 hub (served at /), not from the dashboard tab strip —
+           the strip holds only in-page tabs. Contextual deep-links into
+           the log viewer (an agent's logs, a build entry's log) live
+           inside the relevant dashboard content. -->
       </nav>
     </header>
 

hive-c0re/src/dashboard/state_snapshot.rs

@@ -73,12 +73,11 @@ pub(super) struct StateSnapshot {
     /// links each container's config + each approval's commit into the
     /// forge's `agent-configs` repos.
     forge_present: bool,
-    /// Whether the matrix GUI is reachable at `/matrix/`. Sourced from
-    /// `HIVE_MATRIX_GUI_ENABLED` env var (set by the c0re NixOS module
-    /// when `services.hyperhive.deploy.matrix.gui.enable` is on). The gateway
-    /// (`nix/host-modules/hive-gateway/vhosts.nix`) does the actual `/matrix/` static serving;
-    /// this flag is just an availability signal for iris's dashboard
-    /// chrome so the `M4TR1X →` tab doesn't flash when the GUI is off.
+    /// Whether `services.hyperhive.deploy.matrix.gui.enable` is on. Sourced
+    /// from the `HIVE_MATRIX_GUI_ENABLED` env var the c0re NixOS module
+    /// sets. The matrix client itself is served on the homeserver's
+    /// gateway vhost; the hive's `/matrix/` path redirects there
+    /// (`nix/host-modules/hive-gateway/vhosts.nix`).
     matrix_gui_enabled: bool,
     /// Whether `hive-gateway` is in front of this dashboard. Sourced
     /// from the `HIVE_GATEWAY_ENABLED` env var, which the c0re NixOS

nix/host-modules/hive-c0re/environment.nix

@@ -192,10 +192,9 @@ in
   HIVE_MATRIX_API_URL = config.services.hyperhive.swarm.matrix.apiUrl;
 }
 // lib.optionalAttrs config.services.hyperhive.deploy.matrix.gui.enable {
-  # Availability flags read by the dashboard's `/api/state`.
-  # Matrix GUI lives entirely on the gateway nginx (matrix tab
-  # only shows when both flags are on). Gateway routing detail:
-  # docs/networking/gateway.md::Vhost map.
+  # Surfaces as `matrix_gui_enabled` in the dashboard's `/api/state`.
+  # The matrix GUI is served entirely by the gateway nginx. Gateway
+  # routing detail: docs/networking/gateway.md::Vhost map.
   HIVE_MATRIX_GUI_ENABLED = "1";
 }
 // {

nix/host-modules/hive-matrix.nix

@@ -543,8 +543,9 @@ in
       defaultText = lib.literalExpression "config.services.hyperhive.deploy.matrix.enable";
       description = ''
         Serve a matrix web client at `gatewayHost`'s vhost (`chat.<swarm-domain>`
-        by default). Requires `gatewayHost != null`; the gateway itself always runs. When
-        off, the dashboard's `M4TR1X →` tab is hidden. See
+        by default) and add the swarm UI's **Matrix** quick link to it
+        (`services.hyperhive.swarm.controller.links`). Requires
+        `gatewayHost != null`; the gateway itself always runs. See
         `docs/networking/gateway.md` for the discovery flow that lets clients
         auto-find the sub-domain. The client build itself is
         `deploy.matrix.gui.package` — which client, as opposed to whether
@@ -668,12 +669,9 @@ in
 
     # This swarm-ui quick-links entry. Gated on `gui.enable` too, not just
     # `gatewayHost != null`: `/` on that vhost only serves fluffychat
-    # (below) when the GUI is on — otherwise the link would 404. Same
-    # `gatewayHost && gui.enable` condition the removed hive-dashboard
-    # H0M3 tile used to check via `state.matrix_gui_enabled` before this
-    # link became the only way in (docs/web-ui/dashboard.md::M4TR1X
-    # page). See `services.hyperhive.swarm.controller.links`'s
-    # description.
+    # (below) when the GUI is on — otherwise the link would 404. See
+    # docs/swarm/ui.md::Quick links and
+    # `services.hyperhive.swarm.controller.links`'s description.
     services.hyperhive.swarm.controller.links =
       lib.optional (cfg.gatewayHost != null && deployCfg.matrix.gui.enable)
         {