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.

12 files changed, 36 insertions, 0 deletions

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 (