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.

22 files changed, 66 insertions, 0 deletions

diff --git a/internal/calculator/calculator.go b/internal/calculator/calculator.go
index 0212279..ccad360 100644
--- a/internal/calculator/calculator.go
+++ b/internal/calculator/calculator.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package calculator
 
 import (
diff --git a/internal/calculator/calculator_test.go b/internal/calculator/calculator_test.go
index 173a32d..c8f216b 100644
--- a/internal/calculator/calculator_test.go
+++ b/internal/calculator/calculator_test.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package calculator
 
 import (
diff --git a/internal/repl/commands.go b/internal/repl/commands.go
index be49df1..841265b 100644
--- a/internal/repl/commands.go
+++ b/internal/repl/commands.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/commands_test.go b/internal/repl/commands_test.go
index 90fa3e8..102c34b 100644
--- a/internal/repl/commands_test.go
+++ b/internal/repl/commands_test.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/completer.go b/internal/repl/completer.go
index 3f55212..d66a5ef 100644
--- a/internal/repl/completer.go
+++ b/internal/repl/completer.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/completer_test.go b/internal/repl/completer_test.go
index b36c8ec..e64a000 100644
--- a/internal/repl/completer_test.go
+++ b/internal/repl/completer_test.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/handlers.go b/internal/repl/handlers.go
index 622396f..f68e926 100644
--- a/internal/repl/handlers.go
+++ b/internal/repl/handlers.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/history.go b/internal/repl/history.go
index f670dc7..5f50dee 100644
--- a/internal/repl/history.go
+++ b/internal/repl/history.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/prompt.go b/internal/repl/prompt.go
index 656ad6a..37bb23a 100644
--- a/internal/repl/prompt.go
+++ b/internal/repl/prompt.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/repl.go b/internal/repl/repl.go
index 950156b..55fff97 100644
--- a/internal/repl/repl.go
+++ b/internal/repl/repl.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/repl_completer_test.go b/internal/repl/repl_completer_test.go
index f7c4bae..05ef194 100644
--- a/internal/repl/repl_completer_test.go
+++ b/internal/repl/repl_completer_test.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/repl_test.go b/internal/repl/repl_test.go
index 6a8f20d..2200b2a 100644
--- a/internal/repl/repl_test.go
+++ b/internal/repl/repl_test.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/signal.go b/internal/repl/signal.go
index e0a5ca2..3bffd28 100644
--- a/internal/repl/signal.go
+++ b/internal/repl/signal.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
diff --git a/internal/repl/tty.go b/internal/repl/tty.go
index c5e77a1..12992cf 100644
--- a/internal/repl/tty.go
+++ b/internal/repl/tty.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 package repl
 
 import (
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 (
diff --git a/internal/version.go b/internal/version.go
index 24a97ea..2bcc306 100644
--- a/internal/version.go
+++ b/internal/version.go
@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MIT
+// Copyright (c) 2026 Paul Buetow
+
 // Package internal provides version information for the gt application.
 //
 // This package contains internal constants that are used across the gt project.