Adds a built-in `vide.tag()` action that wraps `Instance:AddTag()` /
`Instance:RemoveTag()` so tags can be applied through `create()` props.
It accepts a static string or array of strings, or a reactive source
returning either. The reactive variant diffs the previous and new tag
sets so only the delta is applied. All tags added by the action are
removed when the surrounding scope is destroyed.
Includes mock support for `AddTag`/`RemoveTag`/`HasTag`/`GetTags`,
tests, API reference docs, and a closing note in the actions tutorial.
Co-authored-by: Cursor <cursoragent@cursor.com>
