feat(zshrc): default j=z alias and 30-day omz update interval

zoxide is part of the base install; a short j jump alias matches
common muscle memory. Refresh omz on a monthly cadence instead of
the bi-weekly default.

.zshrc

@@ -38,6 +38,9 @@ fi
 # Uncomment the following line to disable bi-weekly auto-update checks.
 # DISABLE_AUTO_UPDATE="true"
 
+# How often to auto-update (days). Replaces the older UPDATE_ZSH_DAYS variable.
+zstyle ':omz:update' frequency 30
+
 # Uncomment the following line to change how often to auto-update (in days).
 # export UPDATE_ZSH_DAYS=13
 
@@ -286,5 +289,7 @@ export FZF_DEFAULT_OPS="--extended"
 
 # Initialize zoxide (smarter cd command)
 command -v zoxide &> /dev/null && eval "$(zoxide init zsh)"
+# Short jump alias; expands at use time after zoxide defines `z`.
+alias j="z"
 
 alias k="k -h"       # show human readable file sizes, in kb, mb etc