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

@@ -3,7 +3,7 @@ description: 'Executes a shell command/script on a remote server via SSH on a cu
 
 inputs:
   private_key:
-    description: 'Private SSH Key'
+    description: 'Private SSH 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
   command:
     description: 'Shell command or multi-line script to run on the remote server'
@@ -36,7 +36,12 @@ runs:
         # 1. Setup the SSH directory and private key
         mkdir -p ~/.ssh
         chmod 700 ~/.ssh
-        echo "$SSH_KEY" > ~/.ssh/ssh_command_key
+        #    The key is expected base64-encoded so it's a single-line secret the runner can mask;
+        #    raw multi-line PEM keys are still accepted for backwards compatibility.
+        case "$SSH_KEY" in
+          *"-----BEGIN"*) echo "$SSH_KEY" > ~/.ssh/ssh_command_key ;;
+          *) printf '%s' "$SSH_KEY" | tr -d '[:space:]' | base64 -d > ~/.ssh/ssh_command_key ;;
+        esac
         chmod 600 ~/.ssh/ssh_command_key
 
         # 2. Configure SSH for the custom port/user and bypass host key prompt