Polish docs and repo hygiene

CONTRIBUTING.md

@@ -0,0 +1,22 @@
+# Contributing
+
+Thanks for helping improve ps1-craft. Keep changes small, readable, and fast.
+
+## Style rules
+
+- Keep the prompt hot path fast (no external commands unless feature-gated).
+- Use `printf` over `echo` for formatting.
+- Quote variables and paths; avoid word-splitting.
+- Maintain bash 3.2 compatibility for macOS.
+
+## Add a new theme or palette
+
+1) Update `dotfiles/ps1/ps1.sh` with the new colors/icons.
+2) Add a screenshot under `images/themes/`.
+3) Document it in `library.md` and `README.md`.
+
+## Testing
+
+- Run the installer script you changed.
+- Open a new shell and verify the prompt renders.
+- Run ShellCheck on `dotfiles/**/*.sh`.