Fix ralph-loop unbound variable crash on empty PROMPT_PARTS

[cha1mers]

Summary

• Fixes PROMPT_PARTS[*]: unbound variable crash in setup-ralph-loop.sh when no prompt arguments are provided
• One-line change: ${PROMPT_PARTS[*]} → ${PROMPT_PARTS[*]:-}

Root Cause

The script uses set -euo pipefail (line 6), and set -u treats empty bash arrays as unbound. PROMPT_PARTS=() is initialized on line 9, but ${PROMPT_PARTS[*]} on line 113 triggers the error before reaching the proper empty-prompt validation on line 116.

Test plan

# Before fix — crashes
bash -c 'set -euo pipefail; PROMPT_PARTS=(); PROMPT="${PROMPT_PARTS[*]}"; echo "$PROMPT"'
# → bash: PROMPT_PARTS[*]: unbound variable

# After fix — works, reaches validation
bash -c 'set -euo pipefail; PROMPT_PARTS=(); PROMPT="${PROMPT_PARTS[*]:-}"; echo "result: [$PROMPT]"'
# → result: []

Fixes #432

🤖 Generated with Claude Code

[github-actions]

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