perf(completion): let Oh My Zsh own initialization

A single completion setup avoids duplicate initialization. Homebrew already exports its prefix during environment setup, so resolving it again adds an unnecessary subprocess.

.zshrc

@@ -40,13 +40,10 @@ plugins=(
 
 fpath+=$ZSH/custom/plugins/zsh-completions/src
 
-if [[ "$OSTYPE" == "darwin"* ]] && command -v brew &> /dev/null; then
-    FPATH="$(brew --prefix)/share/zsh/site-functions:${FPATH}"
+if [[ "$OSTYPE" == "darwin"* && -n "$HOMEBREW_PREFIX" ]]; then
+    fpath=("$HOMEBREW_PREFIX/share/zsh/site-functions" ${fpath:#$HOMEBREW_PREFIX/share/zsh/site-functions})
 fi
 
-mkdir -p ~/.cache/zsh
-autoload -U compinit && compinit -C -d ~/.cache/zsh/.zcompdump
-
 SAVEHIST=50000
 
 cheat() {