fix(ralph-loop): use env bash shebang for macOS compatibility

[marushomurai]

Summary

• Change #!/bin/bash to #!/usr/bin/env bash in setup-ralph-loop.sh and stop-hook.sh
• Fixes crash on macOS where /bin/bash is Bash 3.2 (GPLv2), which treats empty arrays as "unbound variable" under set -u

Problem

macOS ships with Bash 3.2.57 at /bin/bash (Apple refuses to ship GPLv3-licensed Bash 4+). This version has a known bug:

set -euo pipefail
ARR=()
echo "${ARR[*]}"  # → "unbound variable" error on Bash 3.2
                   # → works fine on Bash 4.4+

setup-ralph-loop.sh line 113 hits this exact issue when PROMPT_PARTS is empty, causing:

PROMPT_PARTS[*]: unbound variable

Fix

Using #!/usr/bin/env bash resolves the PATH to a modern Bash (e.g. Homebrew's Bash 5.x) when available. This is consistent with the approach already used by explanatory-output-style and learning-output-style hooks in this repo.

Test plan

• Verified on macOS (Bash 3.2 → Bash 5.3.9 via Homebrew)
• setup-ralph-loop.sh with no arguments: shows proper error message instead of crashing
• setup-ralph-loop.sh with arguments: activates loop correctly
• No behavior change on Linux (where /bin/bash is already 5.x)

🤖 Generated with Claude Code

[github-actions]

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