feat(updater): add cooldown option to delay applying updates (#13813)

Introduces a new `zstyle ':omz:update' cooldown <days>` setting that limits
the updater to only apply commits that are at least N days old. Defaults to 0
(current behavior — always pull latest).

When cooldown is set, the updater fetches the remote branch and finds the most
recent commit whose committer timestamp is at least N days old, then applies it
via `git merge --ff-only`. If the local copy is already at or past the cooldown
ref, nothing changes.

- tools/upgrade.sh: reads cooldown zstyle, replaces git pull with fetch +
  merge --ff-only when cooldown > 0
- README.md: documents the new setting under "Getting Updates"
- templates/zshrc.zsh-template: adds commented-out cooldown example alongside
  frequency, with rephrased comments to clarify how the two work together

tools/upgrade.sh

@@ -231,6 +231,10 @@ local ret=0
 remote=${"$(git config --local oh-my-zsh.remote)":-origin}
 branch=${"$(git config --local oh-my-zsh.branch)":-master}
 
+# cooldown: minimum age (in days) of commits to apply
+local cooldown_days
+zstyle -s ':omz:update' cooldown cooldown_days || cooldown_days=0
+
 # repository state
 last_head=$(git symbolic-ref --quiet --short HEAD || git rev-parse HEAD)
 # checkout update branch
@@ -242,7 +246,20 @@ last_commit=$(git rev-parse "$branch")
 if [[ $verbose_mode != silent ]]; then
   printf "${BLUE}%s${RESET}\n" "Updating Oh My Zsh"
 fi
-if LANG= git pull --quiet --rebase $remote $branch; then
+if {
+  if (( cooldown_days > 0 )); then
+    zmodload zsh/datetime
+    local cutoff_epoch cooldown_ref
+    cutoff_epoch=$(( EPOCHSECONDS - cooldown_days * 86400 ))
+    LANG= git fetch --quiet $remote $branch && {
+      cooldown_ref=$(git log --format="%H %ct" "$remote/$branch" \
+        | awk -v c="$cutoff_epoch" '$2 <= c { print $1; exit }')
+      [[ -z "$cooldown_ref" ]] || LANG= git merge --ff-only --quiet "$cooldown_ref"
+    }
+  else
+    LANG= git pull --quiet --rebase $remote $branch
+  fi
+}; then
   # Check if it was really updated or not
   if [[ "$(git rev-parse HEAD)" = "$last_commit" ]]; then
     message="Oh My Zsh is already at the latest version."