DEVX-163: docs: add vikunja-tasks skill and skill validation tests, fix create-task docs

.devin/skills/testing-and-debugging/SKILL.md

@@ -3,6 +3,17 @@
 Make targets for testing, debugging, and CI investigation. **Use these
 instead of raw `pytest`, `ruff`, or `actionlint` commands.**
 
+## When to Invoke
+
+Invoke this skill when running tests, investigating CI failures, or
+linting before push. Also invoke when asked to "run tests", "check
+coverage", or "debug a failure".
+
+## Prerequisites
+
+- `.venv` exists (run `make setup` if not)
+- Tools installed (run `make install-tools` for actionlint/act_runner)
+
 ## Why Make Targets
 
 Make targets encapsulate the correct venv activation, PYTHONPATH, env