Guard commit messages against session metadata

[yoshifuminakamura]

Summary

• add a Repository Policy check for external AI/tooling session metadata in newly introduced commit messages
• document the current Repository Policy checks, including PR base freshness
• document the existing warning-level Shellcheck gate for changed app scripts

Validation

• python3 -m py_compile scripts/tests/check_commit_messages.py scripts/tests/check_pr_base_current.py scripts/tests/check_text_integrity.py
• python3 scripts/tests/check_commit_messages.py
• python3 scripts/tests/check_text_integrity.py
• git diff --check