Fix .gitignore swallowing the pipeline build package

The unanchored 'build/' pattern matched src/apworld_tester/pipeline/build/,
so the package was never committed and a fresh clone failed at import with
ModuleNotFoundError. Anchor build/ and dist/ to the repo root and add the
missing module files. Also ignore .venv/.

.gitignore

@@ -21,6 +21,9 @@ failed_apworlds.txt
 CLAUDE.md
 
 # Build output.
-build/
-dist/
+/build/
+/dist/
 *.egg-info/
+
+# Virtual environment.
+.venv/