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/vikunja-tasks/SKILL.md

@@ -0,0 +1,74 @@
+# vikunja-tasks
+
+Vikunja task lifecycle beyond `create`: querying status, closing, and
+recovering when the tracker is unreachable.
+
+## When to Invoke
+
+- Creating, closing, or checking a Vikunja task
+- A spec workflow step needs the task ID or done state
+- `vikunja.oblachno.oblachno.fyi` fails to resolve / times out
+
+## Prerequisites
+
+- `.env` with `VIKUNJA_TOKEN`
+- Project ID comes from `[tool.devx]` in `pyproject.toml`
+  (`DEVX_VIKUNJA_PROJECT_ID`)
+
+## Create
+
+`make create-task` does **not** forward arguments — call the module:
+
+```bash
+.venv/bin/python -m devx.tools.create_task \
+  --title "Task title (no DEVX-N prefix)" \
+  --description "<h2>Context</h2><p>...</p>"
+```
+
+Prints `DEVX-N` + next steps. Title must not include the task-ID
+prefix (auto-merge prepends it; a manual prefix double-prefixes the
+PR title and fails validation).
+
+## Query / Close
+
+```bash
+# Task details (ID = numeric part of DEVX-N)
+curl -sf -H "Authorization: Bearer $VIKUNJA_TOKEN" \
+  "https://vikunja.oblachno.oblachno.fyi/api/v1/tasks/<N>"
+
+# Close: mark done
+curl -sf -X POST -H "Authorization: Bearer $VIKUNJA_TOKEN" \
+  -H "Content-Type: application/json" -d '{"done":true}' \
+  "https://vikunja.oblachno.oblachno.fyi/api/v1/tasks/<N>"
+```
+
+Post-merge automation marks the task done when the PR squash-merges —
+manual close is only needed for abandoned/superseded tasks.
+
+## Task-ID / Spec Collisions
+
+Vikunja IDs can collide with historical spec files (an old task reused
+the number). Convention: preserve the old file as
+`docs/specs/<ID>-<topic>-historical.md`, then write the new spec at
+`docs/specs/<ID>.md`. Check `git log` on the existing spec before
+moving it.
+
+## Tracker Unreachable
+
+If the Vikunja host fails DNS/TLS:
+
+1. Don't block the whole workflow — record the intended task title in
+   the spec draft and retry `create_task` before branching.
+2. Never invent an ID — branch/PR titles must match a real task or
+   `pre_push_check` / auto-merge validation fails.
+3. DNS failures observed so far were transient; retry after a few
+   minutes before escalating.
+
+## Common Mistakes
+
+- `make create-task -- --title ...` — args are dropped; use the module
+  call above (forwarding fix is S11 scope).
+- Including `DEVX-N:` in the task title — double prefix breaks
+  auto-merge.
+- Closing a task whose PR is still open — auto-merge's post-merge
+  step handles the close; manual close confuses the audit trail.