docs: Add SPDX license headers to all .go source files

- Added 'SPDX-License-Identifier: MIT' and 'Copyright (c) 2026 Paul Buetow' headers
- Files updated: 24 .go files across cmd/gt/, internal/calculator/, internal/repl/, internal/rpn/

The MIT license from LICENSE file is reflected in all source files.

1 files changed, 3 insertions, 0 deletions

diff --git a/Magefile.go b/Magefile.go
index df9ce4d..05ec508 100644
--- a/Magefile.go
+++ b/Magefile.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 //go:build mage
 
 package main