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

zshrc.zsh

@@ -112,6 +112,11 @@ if (( ${+commands[sheldon]} )); then
   unset sheldon_cache sheldon_toml sheldon_lock
 fi
 
+# Atuin shell history (self-hosted sync: atuin.jonnybarnes.uk)
+if (( ${+commands[atuin]} )); then
+  eval "$(atuin init zsh)"
+fi
+
 # Set the prompt
 # We need zsh git integration
 # Autoload zsh's `add-zsh-hook` and `vcs_info` functions