fix(commit-msg): normalize repo commit subjects

[marcus]

Summary

• add a local commit-msg hook to normalize and validate repo commit subjects
• install both hooks through git rev-parse --git-path hooks so linked worktrees work
• document the normalized format and cover merge-tag exemptions in regression tests

Testing

• make test-commit-msg
• go test ./...
• make install-hooks (from a linked worktree)
• manual scripts/commit-msg.sh checks for valid and invalid subjects