Ensure Prezto submodule is initialized on clone/install

install.sh: use --recurse-submodules on the initial clone, and always
run 'git submodule update --init --recursive' before rake install so a
fresh checkout doesn't leave zsh/prezto empty.

README: document --recurse-submodules for manual clones; mention the
recovery command for existing shallow clones.

Fixes: bad math expression errors and 'no such file or directory:
~/.zprezto/runcoms/zshrc' on fresh shell startup when the Prezto
submodule hadn't been initialized.

install.sh

@@ -62,11 +62,17 @@ elif [ -d "$YADR_DIR" ]; then
   exit 1
 else
   log "Cloning drunkendotfiles"
-  git clone --depth=1 "$REPO_URL" "$YADR_DIR"
+  git clone --recurse-submodules "$REPO_URL" "$YADR_DIR"
 fi
 
 cd "$YADR_DIR"
 
+# 1a. Ensure submodules are initialized (Prezto lives in zsh/prezto as a
+# submodule — without it, ~/.zshrc fails with "no such file or directory:
+# ~/.zprezto/runcoms/zshrc"). Safe to run on fresh and existing clones alike.
+log "Ensuring submodules are initialized"
+git submodule update --init --recursive
+
 # 2. Run YADR's native install (Vim plugins, prezto, etc.)
 if have rake; then
   log "Running YADR rake install"