feat: enable hook-based mode on Windows

[MauroDeryckere]

Summary

• Remove #[cfg(not(unix))] fallbacks in run_default_mode and run_hook_only_mode that forced Windows into legacy CLAUDE.md injection mode
• Make ensure_hook_installed cross-platform by wrapping only the chmod call in #[cfg(unix)]
• Windows users can now run rtk init -g and get the same hook-based setup as macOS/Linux (zero context cost, deterministic rewriting)

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk init -g hook script written and settings.json patched on Windows 11
• Manual testing: PreToolUse hook fires and rewrites commands in a live Claude Code session on Windows (Git Bash)
• Manual testing: rtk rewrite "git status" returns rtk git status on Windows

[CLAassistant]

All committers have signed the CLA.