Add tag() action for CollectionService tags

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>

docs/api/creation.md

@@ -118,6 +118,56 @@ A wrapper for `action()` to listen for property changes.
 
     Runs with an action priority of 1.
 
+## tag()
+
+A wrapper for `action()` to add [CollectionService](https://create.roblox.com/docs/reference/engine/classes/CollectionService)
+tags to an instance.
+
+- **Type**
+
+    ```luau
+    function tag(tag: string): Action
+    function tag(tags: { string }): Action
+    function tag(source: () -> string): Action
+    function tag(source: () -> { string }): Action
+    ```
+
+- **Details**
+
+    Adds the given tag(s) to the instance using `Instance:AddTag()`. When the
+    surrounding scope is destroyed, any tags that were added are removed using
+    `Instance:RemoveTag()`.
+
+    A source (function) can be passed instead of a static value to reactively
+    update the applied tags. When the source updates, tags no longer present
+    are removed and new tags are added; tags present in both the previous and
+    new value are left untouched.
+
+    Multiple `tag()` actions can be composed on the same instance.
+
+    Runs with an action priority of 1.
+
+- **Example**
+
+    Apply static tags:
+
+    ```luau
+    create "Frame" {
+        tag("Clickable"),
+        tag({ "Draggable", "Highlightable" })
+    }
+    ```
+
+    Apply a reactive tag driven by a source:
+
+    ```luau
+    local state = source "idle"
+
+    create "Frame" {
+        tag(function() return "state-" .. state() end)
+    }
+    ```
+
 ## mount() <Badge type="info" text="STABLE"><a href="/vide/api/reactivity-core#Scopes">STABLE</a></Badge>
 
 Runs a function in a new stable scope and optionally applies its result to a