fix: make ypi launcher work on macOS bash 3.2

[BleedingDev]

Summary

Fixes macOS startup failures in the ypi launcher by addressing two shell-compatibility issues:

1. set -u + empty array expansion on Bash 3.2 (PASS_ARGS[@]: unbound variable)
2. Non-portable mktemp template usage on BSD/macOS

Changes

• Use portable temp-file template:
  • mktemp "${TMPDIR:-/tmp}/ypi_system_prompt.XXXXXX"
• Build pi argv incrementally and only append optional arrays when non-empty:
  • avoids "${empty_array[@]}" nounset crash on macOS system Bash 3.2
• Add unit regression test in tests/test_unit.sh:
  • invokes ypi with no pass-through args and asserts it reaches mock pi

Repro verified

On macOS with /bin/bash 3.2:

• ypi no longer crashes with PASS_ARGS[@]: unbound variable
• repeated ypi invocations no longer fail with mktemp ... File exists

Related issues

• Fixes macOS Bash 3.2: ypi crashes on startup with PASS_ARGS[@]: unbound variable #2
• Fixes macOS: ypi fails on startup with mktemp template (...XXXXXX.md: File exists) #1 (launcher mktemp portability issue)

[csaroff]

Real human here:

I can confirm that your fixes worked for me, but I think you could make it slightly more concise :)

[csaroff]

Suggested change

	# Bash 3.2 (macOS) + `set -u` treats `${empty_array[@]}` as unbound.
	# Build argv incrementally so zero-length arrays stay zero-arg.
	PI_ARGS=(pi --system-prompt "$COMBINED_PROMPT")
	if [ ${#YPI_EXT_ARGS[@]} -gt 0 ]; then
	PI_ARGS+=("${YPI_EXT_ARGS[@]}")
	fi
	if [ ${#PASS_ARGS[@]} -gt 0 ]; then
	PI_ARGS+=("${PASS_ARGS[@]}")
	fi
	exec "${PI_ARGS[@]}"
	exec pi --system-prompt "$COMBINED_PROMPT" "${YPI_EXT_ARGS[@]:-}" "${PASS_ARGS[@]:-}"

[BleedingDev]

I'll leave that to real maintainer of the codebase. :) My goal was to just show possible fix. :)