Switch shell history from mcfly to self-hosted Atuin

Sync is now self-hosted at atuin.jonnybarnes.uk, so swap mcfly's
sheldon plugin for atuin's own init hook in zshrc.zsh.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YaYP6TMTB5TzyYSWnpF5ZT

brew.sh

@@ -30,6 +30,7 @@ brew install zsh
 # Install other useful utilities
 brew install ack
 brew install age
+brew install atuin
 brew install awscli
 brew install b3sum
 brew install bat
@@ -48,7 +49,6 @@ brew install gnupg
 brew install hexyl
 brew install jq
 brew install lazygit
-brew install mcfly
 brew install neovim
 brew install p7zip
 brew install pcre