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>

.zshrc

@@ -61,7 +61,6 @@ path_extensions=(
   "$HOME/.pyenv/bin"
   "$HOME/.cargo/bin"
   "$HOME/go/bin"
-  "$HOME/scripts"
 )
 
 # Apply add_to_path to each directory (prepend by default)
@@ -133,8 +132,6 @@ done
 # - $ZSH_CUSTOM/aliases.zsh
 # - $ZSH_CUSTOM/functions.zsh
 
-alias arty="arty.sh"
-alias stacks="stacks.sh"
 type bat >/dev/null 2>&1 && alias cat="bat --paging=never"
 
 function y() {