docs+nix: fix stale certFingerprint/HYPERHIVE_PEERS references (hyperhive#3294)

Follow-up to the plumbing removal: docs/swarm/README.md gets the
biggest rewrite (drops the whole "Fingerprint format" section, fixes
the runtime-effects list, the WireGuard config example + "what the
mesh does" bullet), docs/gateway.md and hive-gateway/options.nix drop
4 "needs no certFingerprint" mentions, swarm-peers-removed.nix's
migration-warning text no longer tells an upgrading operator to carry
a field over that no longer exists, swarm.nix/swarm-wireguard.nix/
swarm-controller.nix/swarm-controller's main.rs get comment fixes
where they described the now-removed HYPERHIVE_PEERS shape. Also
caught one more stale "peer hives" mention in docs/web-ui/README.md's
SW4RM tab description that the first pass on this issue missed.

docs/web-ui/README.md

@@ -18,9 +18,7 @@ The **dashboard** itself (`/dashboard.html`) is where you'll spend most
 of your time. It's a single page with exactly four tabs:
 
 - **SW4RM** — every agent, live. This is the default tab and the one
-  you'll check most. When the hive has peer hives configured, they show
-  up here too, as a card list under the main container list — not a
-  separate tab.
+  you'll check most.
 - **Y3R C4LL** — anything waiting on *you*: pending approvals and
   agent questions. If an agent needs a decision from you, it's here.
 - **P3RM1SS10NS** — what tools and system-level access each agent has.