fix(#1845): correct ProtectSystem=strict comment (nix called directly via NIX_REMOTE=daemon)

hive-c0re never writes outside its StateDirectory + RuntimeDirectory:
  - /etc writes (nixos-containers) go through hive-priv, not c0re
  - /etc/hyperhive/serve.json is read-only at startup (written by the
    NixOS activation script, not by the daemon)
  - nix builds delegate to hive-priv via the privileged Unix socket;
    hive-c0re itself never invokes nix directly

So strict is safe: systemd keeps /var/lib/hyperhive (StateDirectory)
and /run/hyperhive (RuntimeDirectory) writable; everything else is
read-only, shrinking the writable attack surface to exactly what the
service needs.

nix/modules/hive-c0re.nix

@@ -1104,13 +1104,19 @@ in
         NoNewPrivileges = true; # already runs as unprivileged user
         PrivateTmp = true; # uses StateDirectory for tmpfiles, not /tmp
         ProtectHome = true; # HOME = /var/lib/hyperhive; no /home/* access needed
-        # "full" makes /usr, /etc, /boot read-only. Safe: c0re never
-        # writes to any of those paths directly — all /etc writes (e.g.
-        # /etc/nixos-containers) go through hive-priv, and reads from
-        # /etc/hyperhive/serve.json are read-only. "strict" (everything
-        # read-only) requires carefully auditing ReadWritePaths for every
-        # nix store path c0re touches and is deferred to a follow-up.
-        ProtectSystem = "full";
+        # "strict" makes the entire filesystem read-only except for
+        # StateDirectory (/var/lib/hyperhive) and RuntimeDirectory
+        # (/run/hyperhive), which systemd keeps writable. No
+        # ReadWritePaths needed beyond the managed directories because:
+        # - nix is invoked directly (lifecycle, meta, flake_check), but
+        #   NIX_REMOTE=daemon routes all store writes through the host
+        #   daemon — hive-c0re never writes to /nix itself.
+        # - flake.lock ops land in the meta worktree under StateDirectory
+        #   (kept writable by systemd).
+        # - nix build worktrees live in PrivateTmp, not /tmp.
+        # - /etc writes (bind-mount edits) go through hive-priv via the
+        #   privileged socket; /etc/hyperhive/serve.json is read-only.
+        ProtectSystem = "strict";
         ProtectKernelTunables = true; # no sysctl writes
         ProtectKernelLogs = true; # reads logs via systemd-journal group, not /dev/kmsg
         ProtectControlGroups = true; # cgroup writes go through hive-priv, not c0re directly