Add global shell functions for dev shell utilities

[Copilot]

Dev shell utilities required relative paths and could only be invoked from the dotfiles root directory. Users working in project directories needed a way to run set-dev-shell --latest elixir without path gymnastics.

Changes

Shell Functions (modules/zsh/functions.sh)

• Added set-dev-shell and check-shell-versions as globally available functions
• Resolves dotfiles path via ${DOTFILES_PATH:-$HOME/Repos/dot-files} for portability
• Error messages guide users to set DOTFILES_PATH if custom location needed

Documentation

• Updated examples to use global functions instead of relative script paths
• Documented DOTFILES_PATH environment variable override

Testing

• Test suite validates functions work from arbitrary directories
• Uses mktemp -d for safe concurrent execution

Usage

# From any project directory
cd ~/my-elixir-project
set-dev-shell --latest elixir
direnv allow

# Custom dotfiles location
export DOTFILES_PATH=/custom/path
set-dev-shell --list

Direct script invocation (~/Repos/dot-files/utilities/set_dev_shell.sh) remains available for CI/automation contexts.

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