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.

config/exports

@@ -18,7 +18,11 @@
 [ -z "${EDITOR:-}" ] && export EDITOR="nvim"
 [ -z "${VISUAL:-}" ] && export VISUAL="code"
 
-export PATH="$XDG_BIN_HOME:$DOTFILES/local/bin:$XDG_DATA_HOME/cargo/bin:/opt/homebrew/bin:/usr/local/bin:$PATH"
+# Bootstrap: ensure local/bin is on PATH so x-path is available
+PATH="$DOTFILES/local/bin:$PATH"
+# Use x-path to deduplicate PATH entries
+PATH="$(x-path prepend "$XDG_BIN_HOME" "$DOTFILES/local/bin" "$XDG_DATA_HOME/cargo/bin" "/opt/homebrew/bin" "/usr/local/bin")"
+export PATH
 
 if ! command -v msg &> /dev/null; then
   # Function to print messages if VERBOSE is enabled