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/spec-driven-development/SKILL.md

@@ -1,5 +1,14 @@
 # Spec-Driven Development
 
+## When to Invoke
+
+Invoke this skill when starting any change — every PR requires a spec
+at `docs/specs/<TASK-ID>.md` that CI validates before merge.
+
+## Prerequisites
+
+- A Vikunja task ID (`DEVX-N`) — see `vikunja-tasks` skill
+
 ## Overview
 
 Every change starts with a spec. No spec, no code. No code, no PR.