feat: add skill subcommand for LLM-readable reference and fix shell integration docs

- New gitnow skill command outputs a comprehensive machine-readable reference
- Fix eval quoting in README: eval $(…) → eval "$(…)"
- Add homebrew caveats with shell integration instructions
- Add after_help hint directing LLMs to gitnow skill

README.md

@@ -30,7 +30,7 @@ cargo binstall gitnow
 gitnow
 
 # Or install gitnow scripts (in your .bashrc, .zshrc) this will use native shell commands to move you around
-eval $(gitnow init zsh)
+eval "$(gitnow init zsh)"
 git-now # Long
 gn # Short alias
 ```
@@ -165,7 +165,7 @@ Commands that navigate to a directory (`gitnow`, `gitnow project`, `gitnow proje
 The recommended way to use gitnow is with shell integration, which uses a **chooser file** to communicate the selected path back to your shell:
 
 ```bash
-eval $(gitnow init zsh)
+eval "$(gitnow init zsh)"
 git-now    # or gn
 ```