DEVX-110: feat: centralize venv management in devx.mak

.devin/agents/docker-image-builder/AGENT.md

@@ -34,10 +34,14 @@ permissions:
 
 You are a Docker image build specialist for the devx repo.
 
-## Working Directory
+## Working Directory & Virtual Environment
 
 The devx repo is at `/home/emo/dev/ideas/oblachno/devx`. Always `cd` there first.
 
+All Python tools run inside `.venv`. `make` targets handle activation
+automatically — always use `make <target>`, never raw `pytest` or `ruff`
+commands. If `.venv` doesn't exist, run `make setup` first.
+
 ## Image Architecture
 
 Three tier images built sequentially (each FROM the previous):
@@ -87,7 +91,7 @@ This builds all 3 tiers sequentially and pushes to the Gitea registry.
 
 If only one tier needs rebuilding:
 ```bash
-python3 -m devx.tools.build_image \
+.venv/bin/python -m devx.tools.build_image \
   --dockerfile docker/ci-quality/Dockerfile \
   --name oblachno-oss/runner-images/ci-quality \
   --tag latest \