python3 not found on Windows (Git Bash) — codex skills fail on first run

## Bug

The `/codex` skill hardcodes `python3` in its Bash pipe commands for parsing codex JSON output. On Windows (Git Bash / MSYS2), Python installs as `python`, not `python3`, causing:

```
/usr/bin/bash: line 238: python3: command not found
```

The skill fails on first invocation, then sometimes works on retry (likely due to shell environment initialization quirks).

## Affected files

- `codex/SKILL.md.tmpl` — 3 instances (lines ~290, ~434, ~476)

All are the same pattern:
```bash
... | PYTHONUNBUFFERED=1 python3 -u -c "
```

## Suggested fix

Replace bare `python3` with a cross-platform probe:

```bash
PYTHON_CMD=$(command -v python3 2>/dev/null || command -v python 2>/dev/null || echo python)
... | PYTHONUNBUFFERED=1 $PYTHON_CMD -u -c "
```

This tries `python3` first (Linux/macOS), falls back to `python` (Windows), and fails clearly if neither exists.

## Environment

- Windows 11, Git Bash (MSYS2)
- Python 3.11.6 installed as `python` (standard Windows installer)
- gstack v1.26.0.0