Replace buggy bash commit-msg hook with Python implementation - #2640

The bash Utilities/Hooks/kw-commit-msg hook has had a regression since 2024-03-19 (commit 7994b76, "Update KW hooks. Remove gerrit and hook chaining"). That commit removed Gerrit review support and, as a side effect, also deleted the only state transition (else: state=rest) that allowed a commit message body after the mandatory blank second line. Since then, the hook has rejected any commit message with real body content beyond that blank line — every line after it gets re-validated by the "must be empty" check forever.

This explains why so many historical commits with proper multi-paragraph bodies exist in this repo's history: they were made via GitHub's server-side squash/merge (which never runs local hooks), with --no-verify, or in checkouts where pre-commit wasn't installed — never through this hook actually passing.

Fix

Replace the bash script with Utilities/Hooks/commit-msg.py, adapted from ITK's own Python commit-message hook implementation:

• Validates subject line length (8-78 chars, configurable via git config hooks.commit-msg.maxLength) and no leading/trailing whitespace
• Requires a blank second line
• Allows a free-form body afterward (the actual fix)
• Drops ITK's mandatory subject-line prefix convention (BUG:/ENH:/etc.), which SimpleITK does not use
• Fixes a missing return in get_max_length() present in ITK's version
• Drops the kw- prefix from the hook id/name/filename

.pre-commit-config.yaml is updated to invoke it with language: python instead of language: system.

Testing

Manually verified against sample commit messages (multi-paragraph body, subject-only, too-short subject, too-long subject, missing blank second line) — all pass/fail as expected. This commit's own message is a multi-paragraph body, validated by the new hook.