Add zsh config: oh-my-zsh + powerlevel10k, lazy-loaded nvm/pyenv

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.

README.md

@@ -1,2 +1,19 @@
 # dotfiles
-My configuration  configuration files that keep me from flipping a table after logging in.
+
+My configuration files that keep me from flipping a table after logging in.
+
+## Setup
+
+```
+git clone https://github.com/andwati/dotfiles ~/dev/dotfiles
+~/dev/dotfiles/install.sh
+```
+
+Symlinks `zsh/.zshrc`, `zsh/.zshenv`, `zsh/.zprofile`, and `zsh/.p10k.zsh` into
+`$HOME`, and clones oh-my-zsh plus the theme/plugins `.zshrc` expects
+(powerlevel10k, zsh-autosuggestions, zsh-syntax-highlighting,
+zsh-history-substring-search, zsh-completions, you-should-use).
+
+Not installed by the script — grab these separately if a fresh machine needs
+them: `nvm`, `pyenv`, `bun`, `pnpm`, `deno`, `rustup`, `docker`, `fzf`,
+`zoxide`, and a Nerd Font (JetBrains Mono Nerd Font) set as your terminal font.