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/quickstart.txt

@@ -0,0 +1,5 @@
+# Test bd quickstart command
+bd quickstart
+stdout 'Dependency-Aware'
+stdout 'bd init'
+stdout 'bd create'