Cuts shell startup from ~440ms to ~150ms by lazy-loading nvm and pyenv
(previously sourced eagerly on every shell start) and caching oh-my-zsh's
completion rescan, which otherwise reran unconditionally every session.
Also fixes a latent bug where oh-my-zsh's extendedglob option silently
broke nvm's default-version auto-activation.
Adds powerlevel10k, zsh-autosuggestions, zsh-syntax-highlighting,
zsh-history-substring-search, zsh-completions, you-should-use, fzf, and
zoxide. install.sh bootstraps a fresh machine: symlinks these dotfiles
and clones the oh-my-zsh theme/plugins.
