Add Makefile target to detect and clean up broken dotfile symlinks

[xtetsuji]

Problem

When dotfiles are renamed or removed from the repository, broken symlinks can remain in the home directory. These broken symlinks point to non-existent files and can cause confusion or errors.

For example:

• If we rename vimrc to vim_config, the old ~/.vimrc symlink becomes broken
• If we remove a dotfile from the repository, its corresponding ~/.dotfile symlink becomes invalid
• Currently, there's no automated way to detect and clean up these broken symlinks

Proposed Solution

Add a new Makefile target clean-broken-symlinks that:

1. Detects broken dotfile symlinks in the home directory (~/.??*)
2. Filters for repository-related symlinks (pointing to this dotfiles repository)
3. Interactively prompts for each broken symlink deletion
4. Shows clear information about what will be removed

Suggested implementation:

clean-broken-symlinks:
	@echo "Checking for broken dotfile symlinks in home directory..."
	@for link in ~/.??* ; do \
		if [ -L "$$link" ] && ! [ -e "$$link" ] ; then \
			target=$$(readlink "$$link") ; \
			if [[ "$$target" == "$(CURDIR)"/* ]] ; then \
				echo "Found broken symlink: $$link -> $$target" ; \
				read -p "Remove $$link? [y/N]: " confirm ; \
				if [ "$$confirm" = "y" ] || [ "$$confirm" = "Y" ] ; then \
					rm -v "$$link" ; \
				fi ; \
			fi ; \
		fi ; \
	done

Features:

• Safe: Only targets symlinks pointing to this dotfiles repository
• Interactive: Asks for confirmation before each deletion
• Informative: Shows what symlink points where
• Non-destructive: Skips regular files and valid symlinks

Usage:

make clean-broken-symlinks

This would help maintain a clean home directory and prevent issues caused by stale symlinks.