Remove dark-mode-notify, document the native approach

The launchd agent ran `pkill -usr1 zsh` on every appearance change, which made
every shell re-source .zshrc. That could never have achieved its goal: the tmux
status bar belongs to the tmux server and colours to a running nvim, so
re-sourcing a shell rc cannot retheme either. The one variable it set,
MACOS_APPEARANCE, was read by nothing. Its only real effect was firing on each
unlock — 5253 times on this machine, 3437 on the iMac — which is what drove the
~/.gitconfig.lock contention fixed in 583bbaa.

The agent and plist are removed from both machines; this drops the script.
ghostty, bat, delta and nvim all handle appearance natively, and tmux 3.6+
detects it over OSC 2031 (#{client_theme} already reports correctly). Wiring the
tmux client-light-theme/client-dark-theme hooks is left as the next step.

Also refreshes the stale parts of the README: the intro still explained
~/.gitconfig as a copy because .extra edited it, and the .extra example still
showed the `git config --global` calls that caused the lock errors, plus the
misspelled GIT_COMMITER_* vars. Following it would have reinstated the bug.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

zsh/dark-mode-notify.zsh

@@ -1,23 +0,0 @@
-#!/usr/bin/env zsh
-
-function switch-dark-mode()
-{
-  local darkMode=true;
-
-  if [[ $(defaults read -g AppleInterfaceStyle 2> /dev/null) != 'Dark' ]]; then
-    darkMode=false
-  fi
-
-  if [[ $darkMode == true ]]; then
-    echo "Switched to dark mode"
-    export MACOS_APPEARANCE="dark"
-  else
-    echo "Switched to light mode"
-    export MACOS_APPEARANCE="light"
-  fi
-
-  # Reload zshrc
-  pkill -usr1 zsh
-}
-
-switch-dark-mode