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.

2 files changed, 6 insertions, 0 deletions

diff --git a/cmd/gt/main.go b/cmd/gt/main.go
index c571792..eccabf7 100644
--- a/cmd/gt/main.go
+++ b/cmd/gt/main.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 // Package gt provides a command-line percentage calculator with RPN support.
 //
 // gt is a versatile calculator that supports both percentage calculations and
diff --git a/cmd/gt/main_test.go b/cmd/gt/main_test.go
index df4d59f..3ad91ab 100644
--- a/cmd/gt/main_test.go
+++ b/cmd/gt/main_test.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package main
 
 import (