[taigrr]

Summary

• Require at least one non-whitespace character in conventional commit descriptions.
• Cover whitespace-only descriptions with scopes and breaking markers, while preserving meaningful descriptions with surrounding spaces or tabs.
• Document the description requirement. Merge and fixup/squash exemptions are unchanged.

Validation

• Regression rejects the original implementation (whitespace-only scoped description incorrectly exits successfully).
• All four shell regression suites pass.
• ShellCheck passes for all hooks, install/uninstall scripts, and tests.
• git diff --check passes.

Independent of pending staged-file-size PR #12; no shared files changed.