feat: add cross-host SSH trust, state-aware teardown, and configurable migration polling

- Add setup/cross_host_ssh.sh to establish ed25519 SSH trust between
  Unraid and Fedora (required by backup/restore scripts for direct SCP)
- Add ssh_key and authorized_key cleanup handlers to setup/cleanup.sh
- Rewrite phase8 cutover to mark GitHub repos as mirrors instead of
  archiving them (archived repos reject push mirror writes), with a
  JSON state snapshot of pre-cutover settings (description, homepage,
  wiki, projects, Pages) for exact restoration on teardown
- Rewrite phase8 teardown to restore from state snapshot with fallback
  to legacy "— was:" description parsing
- Make migration polling configurable via MIGRATION_POLL_INTERVAL_SEC
  and MIGRATION_POLL_TIMEOUT_SEC in .env (was hardcoded 120s/3s)
- Fix preflight SSL validation: check SSL_MODE instead of always
  requiring SSL_EMAIL, add conditional checks per SSL_MODE
- Add preflight checks 23-24: cross-host SSH connectivity
- Add --start-from range validation and cross_host_ssh.sh to run_all.sh

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

run_all.sh

@@ -26,16 +26,22 @@ require_local_os "Darwin" "run_all.sh must run from macOS (the control plane)"
 # ---------------------------------------------------------------------------
 SKIP_SETUP=false
 START_FROM=0
+START_FROM_SET=false
 
 for arg in "$@"; do
   case "$arg" in
     --skip-setup) SKIP_SETUP=true ;;
     --start-from=*)
       START_FROM="${arg#*=}"
+      START_FROM_SET=true
       if ! [[ "$START_FROM" =~ ^[0-9]+$ ]]; then
         log_error "--start-from must be a number (1-9)"
         exit 1
       fi
+      if [[ "$START_FROM" -lt 1 ]] || [[ "$START_FROM" -gt 9 ]]; then
+        log_error "--start-from must be between 1 and 9"
+        exit 1
+      fi
       ;;
     --help|-h)
       cat <<EOF
@@ -121,8 +127,13 @@ if [[ "$SKIP_SETUP" == "false" ]] && [[ "$START_FROM" -eq 0 ]]; then
   run_step "Setup MacBook" "setup/macbook.sh"
   run_step "Setup Unraid" "setup/unraid.sh"
   run_step "Setup Fedora" "setup/fedora.sh"
+  run_step "Cross-host SSH trust" "setup/cross_host_ssh.sh"
 else
-  log_info "Skipping setup (--skip-setup or --start-from=${START_FROM})"
+  if [[ "$START_FROM_SET" == "true" ]]; then
+    log_info "Skipping setup (--skip-setup or --start-from=${START_FROM})"
+  else
+    log_info "Skipping setup (--skip-setup)"
+  fi
 fi
 
 # ---------------------------------------------------------------------------