remove certFingerprint + HYPERHIVE_PEERS plumbing (hyperhive#3294)

Mara wanted the underlying plumbing gone too, not just the dashboard
display. Traced every consumer before cutting:

- certFingerprint (services.hyperhive.swarm.hives.<name>.certFingerprint):
  removed the nix option entirely. Its only consumer was the dashboard
  code removed in the previous commits.
- HYPERHIVE_PEERS: removed entirely — the env var itself, the whole
  block that built it in hive-c0re/environment.nix, and its entry in
  meta.rs's FORWARDED_VARS (which forwarded it into every agent
  container). Turned out to have zero real consumers, not just one:
  the docs claimed hive-agent::identity::peers() read it for qualified
  agent labels, but no such function exists — identity.rs only
  qualifies THIS agent's own label with HYPERHIVE_HIVE_DOMAIN, nothing
  peer-list-related. Grepped the whole hive-agent crate to confirm
  before removing.

services.hyperhive.swarm.peerHives (the nix option HYPERHIVE_PEERS was
built from) is untouched — swarm-wireguard.nix reads it directly for
the wg-hive mesh, a real and unrelated consumer.

Verified: cargo build/clippy/test -p hive-c0re -p swarm-controller all
clean (needed nix develop -c per the usual -lsqlite3 gap), all touched
nix files pass nix-instantiate --parse, and a throwaway nixosSystem
eval confirms the wireguard mesh still configures a peer's
wireguardAddress into wg-hive correctly with certFingerprint gone.

hive-c0re/src/meta.rs

@@ -703,7 +703,6 @@ const FORWARDED_VARS: &[&str] = &[
     "HIVE_FORGE_URL",
     "HIVE_FORGE_PUBLIC_URL",
     "HIVE_MATRIX_URL",
-    "HYPERHIVE_PEERS",
     "HYPERHIVE_HIVE_DOMAIN",
     "HYPERHIVE_HIVE_NAME",
     "HYPERHIVE_SWARM_NAME",