chore(mise): update configs and add cleanup script documentation

Update tooling configs, shell settings, and editor configurations
as part of the mise migration. Add companion documentation for the
cleanup-old-version-managers script and remove unused fish completions.

hosts/s/config/git/overrides/ignore

@@ -22,7 +22,6 @@ dfm.sh
 .phpactor.json
 .zsh_history
 
-**/.claude/settings.local.json
 **/.claude/*
 **/.serena/*
 **/docs/plans/*