Accept the SSH private key base64-encoded

Multi-line secrets aren't masked by the runner, so a raw PEM key passed
as private_key ends up in plain text in the step's env block in the log.
Accept a single-line base64-encoded key instead, which is masked like
any other secret. Raw PEM keys still work for backwards compatibility.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

action.yml

@@ -14,7 +14,7 @@ inputs:
     description: 'SSH Username'
     required: true
   private_key:
-    description: 'SSH Private Key'
+    description: 'SSH Private Key, base64-encoded on a single line (e.g. `base64 -w0 id_ed25519`). A raw PEM key still works but is not masked in the logs.'
     required: true
   transfers:
     description: 'List of transfers formatted as "source_file(s) | remote_destination". One per line.'
@@ -34,7 +34,12 @@ runs:
       run: |
         # 1. Create a secure temporary file for the SSH key
         SSH_KEY_PATH=$(mktemp)
-        printf '%s\n' "$PRIVATE_KEY" > "$SSH_KEY_PATH"
+        #    The key is expected base64-encoded so it's a single-line secret the runner can mask;
+        #    raw multi-line PEM keys are still accepted for backwards compatibility.
+        case "$PRIVATE_KEY" in
+          *"-----BEGIN"*) printf '%s\n' "$PRIVATE_KEY" > "$SSH_KEY_PATH" ;;
+          *) printf '%s' "$PRIVATE_KEY" | tr -d '[:space:]' | base64 -d > "$SSH_KEY_PATH" ;;
+        esac
         chmod 600 "$SSH_KEY_PATH"
 
         # 2. Add Host to known_hosts to prevent verification prompts