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

README.md

@@ -466,6 +466,15 @@ zstyle ':omz:update' frequency 7
 zstyle ':omz:update' frequency 0
 ```
 
+By default, updates always pull the latest changes. If you'd rather let others kick the tires first
+before an update reaches your machine, you can set a cooldown (in days). You'll still get everything —
+just a little later:
+
+```sh
+# Only apply updates that are at least 10 days old
+zstyle ':omz:update' cooldown 10
+```
+
 ### Updates Verbosity
 
 You can also limit the update verbosity with the following settings: