[Task 5] Fix lefthook.yml to preserve user commit messages

[arittr]

Implements:

• Updated prepare-commit-msg hook to check {2} parameter
• Hook only runs commitment when {2} is empty (regular commits)
• Preserves messages for git commit -m and merge commits
• Updated hook to use commands instead of jobs for consistency
• Added explanatory comments matching example hooks

Acceptance criteria met:
✅ lefthook.yml checks {2} parameter before running commitment
✅ git commit generates message (hook runs)
✅ git commit -m "test" preserves message (hook skips)
✅ Merge commits preserve messages (hook skips)
✅ commitment dogfooding still works
✅ Examples already correct (no changes needed)

[arittr]

This change is part of the following stack:

• docs(spec): add lightweight logger interface specification #74
  • feat: implement logger interface with console and silent implementations #75
    • [claude] Starting commit message generation #76
      • 🤖 Generating commit message with claude... #77
        • refactor(eval): inject logger dependency into evaluators and runners #78
          • [Task 5] Fix lefthook.yml to preserve user commit messages #79 ◀

_{Change managed by git-spice.}