Fix --git-since live sync not detecting changes and creating duplicates

Two issues prevented --git-since from working correctly during live sync:

1. Server: File changes weren't detected because git-filtered project nodes
   had empty relevant_paths, so the change processor couldn't map VFS events
   back to tree instances. Fixed by registering $path directories and the
   project folder in relevant_paths even when filtered.

2. Plugin: When a previously-filtered file was first acknowledged, it appeared
   as an ADD patch. The plugin created a new instance instead of adopting the
   existing one in Studio, causing duplicates. Fixed by checking for untracked
   children with matching Name+ClassName before calling Instance.new.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugin/src/Reconciler/reify.lua

@@ -41,14 +41,41 @@ function reifyInstanceInner(unappliedPatch, deferredRefs, instanceMap, virtualIn
 		invariant("Cannot reify an instance not present in virtualInstances\nID: {}", id)
 	end
 
-	-- Instance.new can fail if we're passing in something that can't be
-	-- created, like a service, something enabled with a feature flag, or
-	-- something that requires higher security than we have.
-	local createSuccess, instance = pcall(Instance.new, virtualInstance.ClassName)
+	-- Before creating a new instance, check if the parent already has an
+	-- untracked child with the same Name and ClassName. This enables "late
+	-- adoption" of instances that exist in Studio but weren't in the initial
+	-- Rojo tree (e.g., when using --git-since filtering). Without this,
+	-- newly acknowledged files would create duplicate instances.
+	local adoptedExisting = false
+	local instance = nil
 
-	if not createSuccess then
-		addAllToPatch(unappliedPatch, virtualInstances, id)
-		return
+	for _, child in ipairs(parentInstance:GetChildren()) do
+		local accessSuccess, name, className = pcall(function()
+			return child.Name, child.ClassName
+		end)
+
+		if accessSuccess
+			and name == virtualInstance.Name
+			and className == virtualInstance.ClassName
+			and instanceMap.fromInstances[child] == nil
+		then
+			instance = child
+			adoptedExisting = true
+			break
+		end
+	end
+
+	if not adoptedExisting then
+		-- Instance.new can fail if we're passing in something that can't be
+		-- created, like a service, something enabled with a feature flag, or
+		-- something that requires higher security than we have.
+		local createSuccess
+		createSuccess, instance = pcall(Instance.new, virtualInstance.ClassName)
+
+		if not createSuccess then
+			addAllToPatch(unappliedPatch, virtualInstances, id)
+			return
+		end
 	end
 
 	-- TODO: Can this fail? Previous versions of Rojo guarded against this, but
@@ -96,7 +123,9 @@ function reifyInstanceInner(unappliedPatch, deferredRefs, instanceMap, virtualIn
 		reifyInstanceInner(unappliedPatch, deferredRefs, instanceMap, virtualInstances, childId, instance)
 	end
 
-	instance.Parent = parentInstance
+	if not adoptedExisting then
+		instance.Parent = parentInstance
+	end
 	instanceMap:insert(id, instance)
 end