[codex] Handle Windows python3 stubs

[justinjoy]

Summary

Fixes #144 by removing the plugin's hard dependency on a literal python3 command when running factlog's deterministic scripts and hooks.

What Changed

• Added tools/factlog_python.sh, a Python 3.11+ resolver that tries $FACTLOG_PYTHON, python3, python, then py, and verifies each candidate by executing it.
• Updated the factlog hook scripts to use the resolver instead of calling python3 directly.
• Kept the gate fail-closed when no usable Python is available.
• Updated the factlog skill commands to invoke bundled scripts through the resolver.
• Documented the Windows Microsoft Store python3 stub issue and FACTLOG_PYTHON workaround in both READMEs.
• Added hook regression coverage for a broken python3 stub on PATH.

Root Cause

On Windows, python3 can resolve to the Microsoft Store stub. That command exists, but it cannot run Python code, so plugin commands and hook JSON/path parsing could fail or behave incorrectly even when python or py works.

Validation

• C:\Program Files\Git\bin\bash.exe tests/test_gate_check.sh -> 12 passed, 0 failed
• python -m pytest tests/unit -> 227 passed
• git diff --check -> no issues