feat: add git-safety hook with tests and README docs

[shawndrake2]

Summary

Adds a git-safety PreToolUse hook that blocks destructive git and gh CLI operations:

• Force push protection — blocks git push --force (allows --force-with-lease)
• Protected branch enforcement — prevents commits, merges, rebases, resets, and pushes on main/master
• gh CLI guardrails — blocks gh pr merge, gh pr close, gh issue close, gh repo delete, and gh release delete

Also includes 379 lines of tests covering all patterns, branch-aware rules, and stdin/stdout integration.

Changes

• hook-scripts/pre-tool-use/git-safety.js — the hook
• hook-scripts/tests/pre-tool-use/git-safety.test.js — comprehensive test suite
• README.md — added git-safety to hooks table, removed branch-guard from ideas (now implemented)