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/stats.txt

@@ -0,0 +1,9 @@
+# Test bd stats command
+bd init --prefix test
+bd create 'First issue'
+bd create 'Second issue'
+bd close test-1
+bd stats
+stdout 'Total Issues:'
+stdout 'Open:'
+stdout 'Closed:'