test: add skill validation tests; fix stale make-target refs in skills

- New tests/unit/test_skills_validation.py: structure, make-target,
  file-ref checks + existence tests for all 10 skills
- testing-and-debugging: fix molecule count (6->7), drop
  molecule-all-parallel/pre-push/lint-ci refs (not grm targets),
  add When to Invoke + Prerequisites
- devx-workflow: fix rebase targets (devx-rebase/devx-pr-rebase),
  add When to Invoke + Prerequisites
- spec-driven-development: add When to Invoke + Prerequisites

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

@@ -2,6 +2,16 @@
 
 Quick reference for devx tools when working on this repo.
 
+## 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 |
@@ -12,8 +22,8 @@ Quick reference for devx tools when working on this repo.
 | Check CI status | `make devx-pr-status` or `make devx-pr-status PR=42 WAIT=1` |
 | Fetch CI failure logs | `make devx-pr-logs` or `make devx-pr-logs PR=42 JOB=quality TAIL=50` |
 | Add ready-to-merge label | `make devx-pr-label` or `make devx-pr-label PR=42` |
-| Rebase current branch | `make rebase` |
-| Rebase PR via API | `make pr-rebase` or `make pr-rebase PR=42` |
+| Rebase current branch | `make devx-rebase` |
+| Rebase PR via API | `make devx-pr-rebase` or `make pr-rebase PR=42` |
 
 ## Auto-merge Behavior
 

.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 (`GRM-N`) — see `vikunja-tasks` skill
+
 ## Overview
 
 Every change starts with a spec. No spec, no code. No code, no PR.

.devin/skills/testing-and-debugging/SKILL.md

@@ -3,6 +3,17 @@
 Make targets for testing, debugging, and CI investigation. **Use these
 instead of raw `pytest`, `ruff`, or `molecule` commands.**
 
+## When to Invoke
+
+Invoke this skill when running tests, investigating CI failures, or
+debugging molecule scenarios. Also invoke when asked to "run tests",
+"check coverage", or "debug a failure".
+
+## Prerequisites
+
+- `.venv` exists (run `make setup` if not)
+- For molecule tests: Docker is running
+
 ## Why Make Targets
 
 Make targets encapsulate the correct venv activation, PYTHONPATH, env
@@ -31,9 +42,8 @@ produces false failures (missing dependencies, wrong Python version).
 
 | Task | Command | Notes |
 |------|---------|-------|
-| All scenarios | `make molecule` | All 6 scenarios on Ubuntu 22.04 |
-| All platforms | `make molecule-all` | All 6 scenarios on all 4 OSes |
-| Parallel | `make molecule-all-parallel` | MOLECULE_JOBS=4 |
+| All scenarios | `make molecule` | All 7 scenarios on Ubuntu 22.04 |
+| All platforms | `make molecule-all` | All 7 scenarios on all 4 OSes |
 
 ### Spec-Driven Workflow
 
@@ -46,12 +56,12 @@ CI validates the spec before running expensive jobs.
 **Before pushing any branch:**
 
 ```bash
-make pre-push
+make lint-all && make pytest-cov
 ```
 
-This runs `lint-all` + `pytest-cov`. The pre-push git hook only
-validates the Vikunja task exists — it does NOT run tests. You must
-run `make pre-push` manually.
+This runs all linters + unit tests with coverage. The pre-push git
+hook only validates the Vikunja task exists — it does NOT run tests.
+Run the checks manually (there is no `pre-push` target here).
 
 ## CI Failure Investigation
 
@@ -59,7 +69,7 @@ When investigating a CI failure:
 
 1. **Fetch logs via MCP** — use `mcp_call_tool` with gitea server,
    `actions_run_read` method, `download_job_log` tool
-2. **Reproduce locally** — use `make pytest-cov` or `make lint-ci`
+2. **Reproduce locally** — use `make pytest-cov` or `make lint-all`
    depending on which CI job failed
 3. **Never run raw pytest** — always use the make target