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

## Problem

The OBL-INFRA-548 programme audit found devx lacks a Vikunja
task-lifecycle skill and has no skill validation tests (infra and
sso-bridge have them; grm gained them under GRM-171).
`devx-workflow` documents `make create-task -- --title`, which fails
because `devx-create-task` forwards no arguments.

## Approach

REQ-1: Add `vikunja-tasks` skill: create via module call, query,
  close, spec-collision convention, unreachable-tracker handling.
REQ-2: Add `tests/unit/test_skills_validation.py` covering
  structure, make-target, file-ref checks + existence tests for all
  skills.
REQ-3: Fix broken `make create-task -- --title` documentation in
  `devx-workflow` skill; add missing When to Invoke / Prerequisites
  sections to older-format skills.

Preserve the colliding spec as
[DEVX-163-run-push-stdout-historical](DEVX-163-run-push-stdout-historical.md).

## Test Plan

- `pytest tests/unit/test_skills_validation.py` passes (10 tests).

## Deploy Plan

Documentation/skills only — auto-merge to master; no runtime deploy.

## Rollback Plan

Revert the squash-merge commit; skills are inert documentation.

## Acceptance Criteria

- [x] REQ-1: `vikunja-tasks` skill exists.
- [x] REQ-2: `tests/unit/test_skills_validation.py` exists and passes.
- [x] REQ-3: create-task docs corrected.

## Out of Scope

- Fixing `devx-create-task` argument forwarding (S11 backlog: the
  devx.mak target takes no args; needs env-var or arg forwarding).