feat(updater): add cooldown option to delay applying updates (#13814)

* 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

* fix(updater): address cooldown review feedback

Pass cooldown into upgrade.sh as -c, extract the update helpers, and only
prompt when a cooldown-eligible commit would actually move HEAD.

* fix(updater): follow first-parent history for cooldown

lib/cli.zsh

@@ -903,8 +903,11 @@ function _omz::update {
   }
 
   # Run update script
+  local verbose_mode cooldown_days
   zstyle -s ':omz:update' verbose verbose_mode || verbose_mode=default
-  ZSH="$ZSH" command zsh -f "$ZSH/tools/upgrade.sh" -i -v $verbose_mode || return $?
+  zstyle -s ':omz:update' cooldown cooldown_days || cooldown_days=0
+  [[ $cooldown_days == <-> ]] || cooldown_days=0
+  ZSH="$ZSH" command zsh -f "$ZSH/tools/upgrade.sh" -i -v $verbose_mode -c $cooldown_days || return $?
 
   # Update last updated file
   zmodload zsh/datetime