fix(tools/_lib): /etc/mosaic host-level fallback for credential resolution (#700)

packages/mosaic/framework/tools/_lib/credentials.sh

@@ -15,13 +15,23 @@
 #
 # After loading, service-specific env vars are exported.
 # Run `load_credentials --help` for details.
+#
+# Resolution order (first match wins):
+#   1. $MOSAIC_CREDENTIALS_FILE (explicit override — never second-guessed)
+#   2. $HOME/.config/mosaic/credentials.json
+#   3. /etc/mosaic/credentials.json (host-level fallback)
+# The /etc fallback exists for HOME-redirected profile environments, where
+# $HOME points at a per-profile directory that has no credentials file.
+# Operators symlink /etc/mosaic/credentials.json to the host's canonical
+# file once, instead of exporting MOSAIC_CREDENTIALS_FILE per invocation.
 
 if [[ -z "${MOSAIC_CREDENTIALS_FILE:-}" ]]; then
-  for _cand in "$HOME/.config/mosaic/credentials.json"; do
+  for _cand in "$HOME/.config/mosaic/credentials.json" "/etc/mosaic/credentials.json"; do
     if [[ -f "$_cand" ]]; then MOSAIC_CREDENTIALS_FILE="$_cand"; break; fi
   done
   : "${MOSAIC_CREDENTIALS_FILE:=$HOME/.config/mosaic/credentials.json}"
 fi
+export MOSAIC_CREDENTIALS_FILE
 
 _mosaic_require_jq() {
   if ! command -v jq &>/dev/null; then