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/export.txt

@@ -0,0 +1,10 @@
+# Test bd export command
+bd init --prefix test
+bd create 'Test issue'
+bd export -o export.jsonl
+exists export.jsonl
+
+# Verify the exported file contains the issue
+bd import -i export.jsonl
+bd list
+stdout 'Test issue'