docs+nix: fix stale peer-hives-dashboard references (hyperhive#3294)

Follow-up to the peer-hives removal: docs/web-ui/dashboard.md drops
the whole "P33R H1V3S" section + its stale #peers hash and tab-strip
mentions; docs/swarm/README.md's runtime-effects list and
cross-references point at swarm-ui's roster page instead. Nix comments
in hive-c0re/environment.nix, swarm.nix and swarm-wireguard.nix that
described HYPERHIVE_PEERS/certFingerprint as dashboard-consumed are
updated to say what's actually still true (agent identity only).

Deliberately NOT removed: the `certFingerprint` nix option itself.
It has no remaining Rust consumer, but deleting a documented,
potentially already-set option is a bigger, breaking change than
removing dead display code — flagged in docs/swarm/README.md with a
note instead of silently dropped, so an operator or reviewer can make
that call explicitly rather than have it made for them here.

nix/host-modules/swarm-wireguard.nix

@@ -1,9 +1,9 @@
 # The WireGuard inter-hive mesh for the local host. Split out of
 # ./swarm.nix because the two are different concerns with different
 # audiences: that file declares WHO the peers are (data hive-c0re
-# serialises into HYPERHIVE_PEERS and the dashboard renders), while
-# this one is plain host networking that a machine which runs no hive
-# at all --- the snapshot store, for one --- still needs.
+# serialises into HYPERHIVE_PEERS for agent identity), while this one
+# is plain host networking that a machine which runs no hive at all
+# --- the snapshot store, for one --- still needs.
 #
 # The two stay coupled by data, not by structure: the per-peer
 # `wireguard*` fields live on the peer submodule in ./swarm.nix, since