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/import.txt

@@ -0,0 +1,8 @@
+# Test bd import command
+bd init --prefix test
+bd import -i import.jsonl
+bd show test-99
+stdout 'Imported issue'
+
+-- import.jsonl --
+{"id":"test-99","title":"Imported issue","status":"open","priority":1,"issue_type":"task","created_at":"2025-01-01T00:00:00Z","updated_at":"2025-01-01T00:00:00Z"}