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>

src/tag.luau

@@ -0,0 +1,71 @@
+local action = require "./action"()
+local cleanup = require "./cleanup"
+local effect = require "./effect"
+
+type Tags = string | { string }
+
+local function build_set(value: Tags): { [string]: true }
+    local set: { [string]: true } = {}
+    if type(value) == "string" then
+        set[value] = true
+    else
+        for _, name in value do
+            if type(name) ~= "string" then
+                error("tag() expects a string or an array of strings", 0)
+            end
+            set[name] = true
+        end
+    end
+    return set
+end
+
+local function tag(value: Tags | () -> Tags)
+    return action(function(instance)
+        if type(value) == "function" then
+            local current: { [string]: true } = {}
+
+            effect(function()
+                local new_set = build_set((value :: () -> Tags)())
+
+                for name in current do
+                    if not new_set[name] then
+                        instance:RemoveTag(name)
+                    end
+                end
+
+                for name in new_set do
+                    if not current[name] then
+                        instance:AddTag(name)
+                    end
+                end
+
+                current = new_set
+            end)
+
+            cleanup(function()
+                for name in current do
+                    instance:RemoveTag(name)
+                end
+            end)
+        else
+            local set = build_set(value)
+
+            for name in set do
+                instance:AddTag(name)
+            end
+
+            cleanup(function()
+                for name in set do
+                    instance:RemoveTag(name)
+                end
+            end)
+        end
+    end)
+end
+
+type Action = typeof(action(nil :: any))
+
+return tag :: ((tag: string) -> Action)
+    & ((tags: { string }) -> Action)
+    & ((source: () -> string) -> Action)
+    & ((source: () -> { string }) -> Action)