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

templates/zshrc.zsh-template

@@ -28,9 +28,12 @@ ZSH_THEME="robbyrussell"
 # zstyle ':omz:update' mode auto      # update automatically without asking
 # zstyle ':omz:update' mode reminder  # just remind me to update when it's time
 
-# Uncomment the following line to change how often to auto-update (in days).
+# Uncomment the following line to change the frequency the auto-updater is run (in days).
 # zstyle ':omz:update' frequency 13
 
+# Uncomment the following line to set how old an update must be before it's applied, manually or via the auto-updater (in days).
+# zstyle ':omz:update' cooldown 10
+
 # Uncomment the following line if pasting URLs and other text is messed up.
 # DISABLE_MAGIC_FUNCTIONS="true"