perf(lib): autoload the omz CLI and omz_diagnostic_dump

lib/cli.zsh (944 lines) and lib/diagnostics.zsh (353 lines) were parsed
on every startup to define functions that most shells never call. Move
them to $ZSH/functions/, which is already on $fpath, and autoload the
two entry points from oh-my-zsh.sh instead. The `omz` completion moves
to $ZSH/completions/_omz where compinit picks it up on its own.

Measured on macOS arm64, zsh 5.9: ~1.7 ms less per interactive start.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

oh-my-zsh.sh

@@ -78,6 +78,9 @@ fpath=($ZSH/{functions,completions} $ZSH_CUSTOM/{functions,completions} $fpath)
 # Load all stock functions (from $fpath files) called below.
 autoload -U compaudit compinit zrecompile
 
+# The omz CLI and omz_diagnostic_dump are loaded on first use
+autoload -Uz omz omz_diagnostic_dump
+
 is_plugin() {
   local base_dir=$1
   local name=$2