fix(ha/deploy): use nixos+sudo instead of root SSH; temp key for inter-node comms

Root SSH was failing because only the RSA admin key was authorized but the
local dev box only has an ed25519 key. Fix:

- cluster-config.nix: add ed25519 keys to root (same set as nixos user) so
  future deployments work without the temp-key workaround
- deploy.sh/acceptance-tests.sh: SSH as nixos user with sudo instead of root@
- cluster-init.sh: HA_USER/HA_KEY env vars + n2_ssh()/n2_scp() helpers so
  inter-node SSH works regardless of whether root-to-root is available
- deploy.sh Phase 6: generate temp keypair, authorize on node2, place on node1
  for root to use during cluster-init, clean up afterward

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HaH1cSGvhogRP5ExoF6nD8

modules/ha/cluster-config.nix

@@ -19,9 +19,12 @@
 #   cluster-enable-stonith.sh once the fence key is deployed.
 { lib, vars, ... }:
 {
-  # Root SSH access with the admin key — needed for the deploy script to upload
-  # and run cluster-init.sh as root on node1, and for node1→node2 SSH during init.
-  users.users.root.openssh.authorizedKeys.keys = [ vars.adminSshKey ];
+  # Root SSH access — same key set as nixos user so all admin keys can reach root.
+  users.users.root.openssh.authorizedKeys.keys = [
+    vars.adminSshKey
+    "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICMJhrfFayLBG+gWtO6oAvgambw5nWWgztiTFEaaaVRH debian@surface"
+    "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGygkCljN6uKpdJbHTOQtn8ZnH+wKXDLAwrDFbLrE/65 nixos@nixos"
+  ];
 
   # Passwordless sudo for wheel — operator SSHes as nixos and uses sudo for
   # cluster management commands (drbdadm, crm*, pcs, etc.)