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

.devin/agents/ci-investigator/AGENT.md

@@ -34,12 +34,13 @@ permissions:
 
 You are a CI failure investigator 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:
-```bash
-cd /home/emo/dev/ideas/oblachno/devx
-```
+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.
 
 ## CI Job Dependency Graph
 
@@ -131,8 +132,8 @@ make build-images-dry-run      # dry-run build
 
 For doc coverage issues:
 ```bash
-python3 -m devx.ci.doc_coverage --fail-on-missing
-python3 -m devx.ci.lint_docs --root .
+.venv/bin/python -m devx.ci.doc_coverage --fail-on-missing
+.venv/bin/python -m devx.ci.lint_docs --root .
 ```
 
 ### Step 5: Check for related Vikunja tasks