docs: name the matrix host options by their new path

Every prose and error-message reference to the moved options, including
two in hive-c0re/src/matrix.rs that omitted the services. prefix and so
read as the per-agent option. An error that names an option the operator
cannot find is worse than no hint.

hive-c0re/src/matrix.rs

@@ -1,7 +1,8 @@
 //! Optional matrix-tuwunel wiring: shared registration token (host) +
 //! per-agent UIAA registration → `<agent-state>/matrix-token`. No-op
 //! when the `hive-matrix` container isn't running, so operators who
-//! haven't flipped `hyperhive.matrix.enable = true` pay nothing.
+//! haven't flipped `services.hyperhive.swarm.matrix.enable = true` pay
+//! nothing.
 //!
 //! See `docs/matrix.md::Provisioning flow (registration token)` for
 //! the full UIAA round-trip, token-file shape, and host/container
@@ -16,7 +17,7 @@ use crate::coordinator::Coordinator;
 
 /// Client-server API base this daemon provisions against, from
 /// `HIVE_MATRIX_API_URL` (set by `hive-c0re.nix` from
-/// `hyperhive.matrix.apiUrl`).
+/// `services.hyperhive.swarm.matrix.apiUrl`).
 ///
 /// `None` means **this hive has no homeserver to provision against** and
 /// every matrix path no-ops — see [`is_present`]. There is deliberately no
@@ -43,7 +44,8 @@ fn matrix_http() -> Option<&'static str> {
 /// rather than the missing variable.
 fn matrix_base() -> Result<&'static str> {
     matrix_http().context(
-        "matrix: no homeserver configured (hyperhive.matrix.apiUrl / HIVE_MATRIX_API_URL) — \
+        "matrix: no homeserver configured \
+         (services.hyperhive.swarm.matrix.apiUrl / HIVE_MATRIX_API_URL) — \
          this path should have been gated on matrix::is_present()",
     )
 }