perf: fixed slow zsh startup in new tabs

[CaddyDz]

New iTerm tabs took ~0.6s to show a prompt. Profiling showed .zshrc ran compinit three times per shell (oh-my-zsh, then the Homebrew and Docker Desktop blocks). Each extra call re-audited ~970 completion files, and because fpath differed between calls the completion dump was rewritten twice on every start (~0.45s). A second, intermittent cause: ZSH_COMPDUMP was keyed on $HOST, so macOS flipping the hostname (VPN / DHCP reverse-DNS) rebuilt the whole dump again.

• Put Homebrew's site-functions and ~/.docker/completions on fpath before sourcing oh-my-zsh so its single compinit covers them, and dropped the two trailing compinit blocks
• Replaced the $(brew --prefix) subprocess with ${commands[brew]:h:h}
• Kept Docker Desktop's marker comments so it does not re-append its own compinit block on a future update
• Disabled the oh-my-zsh auto-update check via zstyle ':omz:update' mode disabled: its two git calls through Apple's xcrun shim cost ~45ms per tab; update manually with omz update
• Gave ZSH_COMPDUMP a fixed filename

Startup on a tty (what iTerm spawns) went from 0.61s to 0.11s. Verified that aliases, functions, options, keybindings, prompt, and tab-completion candidates are unchanged, with one exception: docker <TAB> now uses Docker Desktop's generated completer instead of the legacy static script Homebrew links from the app bundle, which was mis-parsing docker --help output into bogus candidates.

[skip ci]