fix: batch actionable community PR fixes

[syhya]

Summary

This PR batches three high-confidence fixes from currently open community submissions and applies them together:

1. Hookify import-path fix (from fix: hookify plugin imports broken by versioned cache path #27796)

   • Switches hookify.core.* imports to core.* in hook scripts and rule engine
   • Fixes No module named 'hookify' when plugin is loaded from versioned cache paths

2. /clean_gone worktree handling fix (from fix: correct worktree handling bugs in clean_gone command #27605)

   • Removes the full two-character branch marker prefix from git branch -v
   • Uses grep -F for literal worktree branch matching
   • Uses -n idiomatic non-empty check and read -r in loop

3. Missing frontmatter field docs (from docs: Add missing frontmatter fields to command reference #27717)

   • Adds name, user-invocable, context, agent, and hooks to command frontmatter reference

Validation

• python -m py_compile on all modified hookify Python files
• Local hookify smoke test in both layouts:
  • versioned cache layout (.../hookify/0.1.0/...)
  • workspace layout (plugins/hookify/...)
• All hook entry points return {} for benign payloads in both layouts

Notes

This intentionally consolidates three small fixes into one mergeable patch to reduce review overhead and move open PR backlog items forward.