Adds guidance for agents writing custom project-specific prek hooks as Python scripts, prompted by review feedback on #1532 where a bash hook was rejected in favor of Python.

New skill usethis-prek-hook-bespoke-create covers:

• Location/naming: .py scripts in hooks/ at project root, kebab-case filenames
• Pure Python constraint: Standard library only, no subprocessing — if it needs an external tool, it's not a bespoke hook
• Script structure: sys.exit conventions, stderr for errors, optional --fix flag via argparse
• Performance: Targeted file reads, efficient glob patterns, avoid heavy imports
• Wiring: Local system hook entry using uv run --frozen --offline hooks/<id>.py

Example hook config from the skill:

- repo: local
  hooks:
    - id: check-skills-documented
      name: check-skills-documented
      entry: uv run --frozen --offline hooks/check-skills-documented.py
      language: system
      always_run: true
      pass_filenames: false
      priority: 0

Defers to existing usethis-prek-add-hook skill for priority assignment and placement.

• Fixes Create an agent skill for writing bespoke git hooks #1540

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