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/init.txt

@@ -0,0 +1,4 @@
+# Test bd init command
+bd init --prefix test
+stdout 'initialized successfully'
+exists .beads/test.db