Change the copyright holder to Coffey Labs

119 SPDX-FileCopyrightText headers and the README's licence line.

The distinction that matters here: LINUXexpert-org appears in this repository
in two completely different roles. As a copyright holder in the SPDX headers,
which is what changes, and as the GitHub organisation in the module path and 64
import statements, which does not -- the repository still lives at
github.com/LINUXexpert-org/stalwart-migrator, and rewriting that would not be a
licence change, it would break the build.

Both replacements are anchored to their copyright forms, so an import path
cannot match either. Import count is 64 before and after, and go.mod is
untouched.

LICENSE untouched: the FSF's copyright on the GPL text and the "<name of
author>" placeholders are not ours to edit.

go vet, go build and go test all clean.

internal/applyplan/tenants.go

@@ -1,4 +1,4 @@
-// SPDX-FileCopyrightText: 2026 LINUXexpert-org
+// SPDX-FileCopyrightText: 2026 Coffey Labs
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 package applyplan