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

docs/specs/DEVX-163.md

@@ -1,33 +1,30 @@
-# DEVX-163: Fix _run_push to check stdout for HTTP 500
+# DEVX-163: Add vikunja-tasks skill and skill validation tests, fix create-task docs
 
 ## Problem
-`_run_push` only checked `result.stderr` for HTTP 500, but docker push
-sends the "received unexpected HTTP status: 500 Internal Server Error"
-message to **stdout**, not stderr. This means the tenacity retry logic
-added in DEVX-162 never triggered — the push failed immediately without
-retrying.
 
-## Approach
-Check both `result.stdout` and `result.stderr` for the "500" status code.
-Also update the "already exists" check in `push_image` to check both
-streams, since docker may send that message to stdout as well.
+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.
 
-REQ-1: _run_push checks both stdout and stderr for HTTP 500
-REQ-2: push_image "already exists" check uses combined stdout+stderr
-REQ-3: All existing tests pass with 100% coverage
+## Requirements
 
-## Test Plan
-- Unit tests for stdout 500 detection
-- Unit tests for stderr 500 detection
-- Manual: trigger build-images workflow and verify retry works
-
-## Deploy Plan
-- Merge to master
-
-## Rollback Plan
-- Revert the merge commit
+- 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.
 
 ## Acceptance Criteria
-- [x] REQ-1: _run_push checks both stdout and stderr for HTTP 500
-- [x] REQ-2: push_image "already exists" check uses combined stdout+stderr
-- [x] REQ-3: All existing tests pass with 100% coverage
+
+- [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).