fix(ralph-wiggum): handle empty PROMPT_PARTS array with set -u

[KCW89]

Summary

Fixes a bash strict mode compatibility bug in the ralph-wiggum plugin that causes the script to crash when invoked without arguments.

Problem

The setup-ralph-loop.sh script uses set -euo pipefail for strict error handling. When invoked without arguments (e.g., /ralph-loop --help or just /ralph-loop), the PROMPT_PARTS array remains empty.

Accessing an empty array with ${PROMPT_PARTS[*]} on line 113 triggers bash's unbound variable error under set -u:

PROMPT_PARTS[*]: unbound variable

This prevents the script from reaching the helpful "No prompt provided" error message.

Solution

Use bash's default value syntax ${PROMPT_PARTS[*]:-} which provides an empty string when the array is unset/empty, satisfying strict mode while allowing the script to continue to proper validation.

Changes

-# Join all prompt parts with spaces
-PROMPT="${PROMPT_PARTS[*]}"
+# Join all prompt parts with spaces (handle empty array with set -u)
+PROMPT="${PROMPT_PARTS[*]:-}"

Testing

• ✅ /ralph-loop without args - now shows helpful usage message instead of crashing
• ✅ /ralph-loop --help - displays help documentation correctly
• ✅ /ralph-loop <task> - works as expected (unchanged behavior)