Add scripttest-based integration tests (#59)

- Complete test coverage for all major bd commands:
  blocked, close, create, dep (add/remove/tree), export, help,
  import, init, list, quickstart, ready, show, stats, update, version
- Test data files validate command behavior and output
- Enables automated testing of full CLI workflows

cmd/bd/testdata/update.txt

@@ -0,0 +1,8 @@
+# Test bd update command
+bd init --prefix test
+bd create 'Issue to update'
+bd update test-1 --status in_progress
+stdout 'Updated issue:'
+
+bd show test-1
+stdout 'in_progress'