feat: move scripts from ~/scripts to ~/bin and track in git

Drop .sh extension — scripts are now invocable directly as arty and stacks.
Remove redundant aliases from .zshrc since ~/bin is already in PATH.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -16,6 +16,10 @@
 !.zshenv
 !.hushlogin
 !arty.yml
+!bin/
+!bin/arty
+!bin/stacks
+!bin/README.md
 !.vscode/
 !.vscode/settings.json
 !.vscode/keybindings.json