Self-clone in bootstrap.sh so curl-pipe works; document quick start

Piping bootstrap.sh into sh left $0 as a non-path, so SCRIPT_DIR pointed
at the caller's cwd and the module scripts couldn't be found (exit 127).
When no checkout is present, clone/update the repo into ~/.dotfiles and
re-exec from there. Add a Quick start section to the README.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bootstrap.sh

@@ -6,7 +6,28 @@
 #   (or, from a checked-out clone: ./bootstrap.sh)
 set -eu
 
+REPO_URL="${DOTFILES_REPO:-https://git.nic-oconnor.com/public/dotfiles.git}"
+DOTFILES_DIR="${DOTFILES_DIR:-$HOME/.dotfiles}"
+
 SCRIPT_DIR="$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)"
 
+# When curl-piped into `sh`, there is no checkout on disk: $0 is not a real
+# path, so the module scripts alongside this file are missing. Clone (or
+# update) the repo, then re-exec bootstrap.sh from the checkout.
+if [ ! -f "$SCRIPT_DIR/zsh/install.sh" ]; then
+  if ! command -v git >/dev/null 2>&1; then
+    echo "bootstrap: git is required to self-clone; install git first" >&2
+    exit 1
+  fi
+  if [ -d "$DOTFILES_DIR/.git" ]; then
+    echo "bootstrap: updating existing checkout at $DOTFILES_DIR"
+    git -C "$DOTFILES_DIR" pull --ff-only
+  else
+    echo "bootstrap: cloning $REPO_URL into $DOTFILES_DIR"
+    git clone "$REPO_URL" "$DOTFILES_DIR"
+  fi
+  exec "$DOTFILES_DIR/bootstrap.sh"
+fi
+
 "$SCRIPT_DIR/zsh/install.sh"
 "$SCRIPT_DIR/git/install.sh"