feat: add get_repo_list() and replace REPO_N_NAME with REPO_NAMES in validation

- Add get_repo_list() helper to split space-delimited REPO_NAMES
- Replace REPO_1_NAME/REPO_2_NAME/REPO_3_NAME with single REPO_NAMES in
  _ENV_VAR_NAMES and _ENV_VAR_TYPES validation arrays

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

lib/common.sh

@@ -123,6 +123,15 @@ require_vars() {
   done
 }
 
+# Split REPO_NAMES (space-delimited) into one name per line.
+# Usage:  while IFS= read -r repo; do ...; done < <(get_repo_list)
+#    or:  read -ra REPOS <<< "$REPO_NAMES"   (for array-style usage)
+get_repo_list() {
+  local repos=()
+  read -ra repos <<< "${REPO_NAMES:-}"
+  printf '%s\n' "${repos[@]}"
+}
+
 # ---------------------------------------------------------------------------
 # .env value validators — shared by configure_env.sh, preflight.sh,
 # bitwarden_to_env.sh. Each returns 0 (valid) or 1 (invalid).
@@ -202,7 +211,7 @@ _ENV_VAR_NAMES=(
   BACKUP_STORAGE_PATH BACKUP_RETENTION_COUNT
   RUNNER_DEFAULT_IMAGE RUNNER_DEFAULT_CAPACITY RUNNER_DEFAULT_DATA_PATH LOCAL_RUNNER_DATA_PATH
   GITHUB_USERNAME GITHUB_TOKEN
-  REPO_1_NAME REPO_2_NAME REPO_3_NAME
+  REPO_NAMES
   MIGRATE_ISSUES MIGRATE_LABELS MIGRATE_MILESTONES MIGRATE_WIKI
   GITHUB_MIRROR_INTERVAL
   NGINX_CONTAINER_NAME NGINX_CONF_PATH SSL_MODE
@@ -222,7 +231,7 @@ _ENV_VAR_TYPES=(
   path integer
   nonempty positive_integer nonempty nonempty
   nonempty nonempty
-  nonempty nonempty nonempty
+  nonempty
   bool bool bool bool
   nonempty
   nonempty path ssl_mode