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

- Add vikunja-tasks: task create/query/close lifecycle
- New tests/unit/test_skills_validation.py: structure, make-target,
  file-ref checks + existence tests for all 8 skills; devx.mak targets
  resolved from src/devx/make/ (editable install)
- devx-workflow/spec-driven-development/testing-and-debugging: add
  When to Invoke + Prerequisites; fix broken create-task syntax
- Preserve colliding spec as DEVX-163-run-push-stdout-historical

Implements: REQ-1 REQ-2 REQ-3

.devin/skills/devx-workflow/SKILL.md

@@ -2,11 +2,21 @@
 
 Quick reference for devx tools when working on the devx repo itself.
 
+## When to Invoke
+
+Invoke this skill when creating PRs, checking CI status, adding
+labels, rebasing branches, or performing any PR lifecycle operation.
+
+## Prerequisites
+
+- `.venv` exists (run `make setup` if not)
+- `.env` with `DEVELOPER_GITEA_API_TOKEN`, `VIKUNJA_TOKEN`
+
 ## PR Workflow (use these, not raw git/tea/MCP)
 
 | Task | Command |
 |------|---------|
-| Create Vikunja task | `make create-task -- --title "..." --description "..."` |
+| Create Vikunja task | `.venv/bin/python -m devx.tools.create_task --title "..." --description "..."` (make target doesn't forward args) |
 | Create PR | `make create-pr` |
 | Push + create PR | `make push-with-pr` |
 | Check CI status | `make devx-pr-status` or `make devx-pr-status PR=42 WAIT=1` |