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

.goreleaser.yaml

@@ -58,3 +58,7 @@ brews:
     description: "Git Now is a utility for easily navigating git projects from common upstream providers."
     install: |
       bin.install "gitnow"
+    caveats: |
+      To enable shell integration, add this to your .zshrc:
+
+        eval "$(gitnow init zsh)"