Convert dotfiles management from symlinks to chezmoi

[jaymcgavren]

Summary

• Rename all managed dotfiles with dot_ prefix so chezmoi places them correctly in ~/
• Add .chezmoiignore to exclude repo-only dirs (bin/, todo-txt-sh/, etc.)
• Add .chezmoi.toml.tmpl with user data for bootstrapping new machines
• Remove bootstrap/link_dotfiles.sh (replaced by chezmoi apply)
• Fix hardcoded paths: gitexcludes path in gitconfig, aliases source in zshrc

Bootstrap on a new machine

git clone https://github.com/jaymcgavren/dotfiles ~/dotfiles
chezmoi init --source ~/dotfiles
chezmoi apply