fix: wire destructive_command_guard plugin into hook engine (#316)

[thomwebb]

Problem

The destructive_command_guard plugin (mpfaffenberger#316) is silently dead code. The plugin loader at code_puppy/plugins/__init__.py:29 requires every built-in plugin to have a register_callbacks.py. This plugin didn't have one — it shipped only detector.py + __init__.py + a passing test suite. The detection patterns were defined, tested, and never wired.

Compare to force_push_guard/ (mpfaffenberger#310): that plugin has detector.py plus register_callbacks.py that calls register_callback("run_shell_command", ...). That's why mpfaffenberger#310 actually works and mpfaffenberger#316 doesn't.

Fix

Added register_callbacks.py to destructive_command_guard/, following the exact same pattern as force_push_guard:

• Hooks into the run_shell_command phase
• Interactive TTY: shows an approval prompt via get_user_approval_async with a Rich panel (pattern name, description, full command)
• Non-interactive (CI/piped/sub-agent): hard-blocks with {"blocked": True} and a descriptive error message
• Auto-registers on import

Verification

• All 72 existing detector tests pass
• Callback confirmed registered in the run_shell_command hook chain
• Safe commands (git status) → None (allowed)
• Destructive commands (rm -rf /) → blocked with reasoning
• Ruff lint + format + isort all pass

Fixes mpfaffenberger#316