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

scripts/ha/acceptance-tests.sh

@@ -24,8 +24,9 @@ RESULTS=()
 pass() { echo "  PASS: $1"; ((PASS++)); RESULTS+=("PASS $1"); }
 fail() { echo "  FAIL: $1"; ((FAIL++)); RESULTS+=("FAIL $1"); }
 
-n1() { ssh -i ~/.ssh/id_ed25519 -o StrictHostKeyChecking=no -o ConnectTimeout=5 "root@${NODE1_IP}" "$@" 2>/dev/null; }
-n2() { ssh -i ~/.ssh/id_ed25519 -o StrictHostKeyChecking=no -o ConnectTimeout=5 "root@${NODE2_IP}" "$@" 2>/dev/null; }
+HA_USER="nixos"
+n1() { ssh -i ~/.ssh/id_ed25519 -o StrictHostKeyChecking=no -o ConnectTimeout=5 "${HA_USER}@${NODE1_IP}" sudo "$@" 2>/dev/null; }
+n2() { ssh -i ~/.ssh/id_ed25519 -o StrictHostKeyChecking=no -o ConnectTimeout=5 "${HA_USER}@${NODE2_IP}" sudo "$@" 2>/dev/null; }
 
 echo "════════════════════════════════════════════════════"
 echo " HA Cluster Acceptance Tests — $(date '+%Y-%m-%d %H:%M:%S')"