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.

7 files changed, 21 insertions, 0 deletions

diff --git a/internal/rpn/number.go b/internal/rpn/number.go
index 98c84a5..277b054 100644
--- a/internal/rpn/number.go
+++ b/internal/rpn/number.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package rpn
 
 import (
diff --git a/internal/rpn/operations.go b/internal/rpn/operations.go
index aeccf68..5231ed6 100644
--- a/internal/rpn/operations.go
+++ b/internal/rpn/operations.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package rpn
 
 import (
diff --git a/internal/rpn/operations_test.go b/internal/rpn/operations_test.go
index 9445df0..02e07c3 100644
--- a/internal/rpn/operations_test.go
+++ b/internal/rpn/operations_test.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package rpn
 
 import (
diff --git a/internal/rpn/rpn.go b/internal/rpn/rpn.go
index a5b7e40..b2a12dc 100644
--- a/internal/rpn/rpn.go
+++ b/internal/rpn/rpn.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package rpn
 
 import (
diff --git a/internal/rpn/rpn_test.go b/internal/rpn/rpn_test.go
index 4e8eb04..e3bf43e 100644
--- a/internal/rpn/rpn_test.go
+++ b/internal/rpn/rpn_test.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package rpn
 
 import (
diff --git a/internal/rpn/variables.go b/internal/rpn/variables.go
index 5214403..efc95ef 100644
--- a/internal/rpn/variables.go
+++ b/internal/rpn/variables.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package rpn
 
 import (
diff --git a/internal/rpn/variables_test.go b/internal/rpn/variables_test.go
index 277a3ea..cb12f40 100644
--- a/internal/rpn/variables_test.go
+++ b/internal/rpn/variables_test.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package rpn
 
 import (