fix(#2860): no loopback default for the matrix homeserver

Third and last of #2860's agent-facing URL fallbacks. The operator's
ruling was "any special casing is done on the nix side - same binaries,
no hard coded fallback", so the default is deleted rather than replaced.

Every layer guessed the same wrong thing, and each guess was only ever
correct for a process sharing the host netns:

- nix/agent-modules/matrix.nix: matrixUrlDefault = localhost:8008, both
  as the option's default and as a sentinel the daemon unit compared
  against to decide whether to write HIVE_MATRIX_URL. Now nullOr str,
  default null, the guard is != null, and the doc says what forge.url's
  already says: null means "no matrix", not "guess one".
- nix/host-modules/hive-c0re/environment.nix: forwarded
  http://127.0.0.1:<port> when no gatewayHost was set. hive-c0re shares
  the host netns so it reads as harmless, but the value is handed to
  agents, which do not -- there it names the agent itself. Now forwarded
  only when there is a gateway vhost to name, matching the guard
  HIVE_MATRIX_PUBLIC_URL already uses twelve lines below.
- hive-matrix-mcp: paths::DEFAULT_HOMESERVER was the same address
  compiled in, so dropping the nix defaults alone would have left the
  daemon dialling loopback inside the agent's own netns -- the very bug,
  one layer down. homeserver_url() is now Option, and an account with no
  homeserver is skipped with a log, exactly as one with no token is.
  discover_token_accounts already refused to guess for the same reason.

Two comments taught the assumption back to the next reader ("shared host
netns means every agent container resolves localhost to the same
machine"); both now say which side of the netns boundary they describe.
MATRIX_HTTP keeps its value -- hive-c0re really does share the host
netns -- but no longer claims agents do.

Gated with nix eval against the extended agent-base config, as a pair:
with no url set the daemon unit carries no HIVE_MATRIX_URL, and with one
set it carries exactly that. Either check alone passes on a broken guard.

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

@@ -100,19 +100,25 @@ in
   # gated on hyperhive being enabled). See `docs/gateway.md::HIVE_FORGE_URL`.
   HIVE_FORGE_URL = "http://${config.services.hyperhive.forge.domain}";
 }
-// lib.optionalAttrs config.services.hyperhive.matrix.enable {
-  # In-cluster matrix homeserver URL for each agent's
-  # hive-matrix-daemon — the gateway vhost (`matrix.<domain>`). The
-  # gatewayHost null-guard falls back to loopback so a domain-less
-  # config still evals. Forwarded to agents by meta.rs alongside
-  # HIVE_FORGE_URL; shares the same env-forwarding ordering caveat
-  # (value baked at config-generation time).
-  HIVE_MATRIX_URL =
-    if config.services.hyperhive.matrix.gatewayHost != null then
-      "http://${config.services.hyperhive.matrix.gatewayHost}"
-    else
-      "http://127.0.0.1:${toString config.services.hyperhive.matrix.httpPort}";
-}
+//
+  lib.optionalAttrs
+    (config.services.hyperhive.matrix.enable && config.services.hyperhive.matrix.gatewayHost != null)
+    {
+      # In-cluster matrix homeserver URL for each agent's
+      # hive-matrix-daemon — the gateway vhost (`matrix.<domain>`).
+      # Forwarded to agents by meta.rs alongside HIVE_FORGE_URL; shares the
+      # same env-forwarding ordering caveat (value baked at
+      # config-generation time).
+      #
+      # A domain-less config forwards nothing rather than falling back to
+      # loopback. The old fallback read as harmless because hive-c0re shares
+      # the host netns — but the value it produced was handed to *agents*,
+      # which do not, so `127.0.0.1` there names the agent itself. An absent
+      # forward leaves `hyperhive.matrix.url` null and the daemon no-ops;
+      # that is the honest answer when the hive has no matrix vhost to point
+      # at.
+      HIVE_MATRIX_URL = "http://${config.services.hyperhive.matrix.gatewayHost}";
+    }
 // lib.optionalAttrs config.services.hyperhive.matrix.gui.enable {
   # Availability flags read by the dashboard's `/api/state`.
   # Matrix GUI lives entirely on the gateway nginx (matrix tab