[workshop-sim] Repair: Add a worked example and prerequisite-skip framing to the GitHub Actions intro

### Problem statement

`04-github-actions-intro.md` is the lowest-scoring step in the entire 30-step curriculum (overall score 5.39/10) and has the third-highest conditional dropout in the simulation (16.2% across 44,173 at-risk runs), with `concept-overload` as the single dominant failure category (7,149 of 7,149 failures at this step). The rubric data shows why: 28 new concepts packed into only 756 words (a very high concept-per-word density), zero prerequisite framing (`has_prereq_section: false`), and an `active_learning` score of only 3.9/10 despite 5 checklist items.

### Proposed change

Split the concept load with one short worked example (e.g., annotate a single trigger → job → step flow end-to-end before introducing the full vocabulary list) and add a brief "Before You Start" prerequisite framing sentence that tells the learner what GitHub Actions terminology they can skip if already familiar (mirroring the "already familiar" skip pattern used successfully in `05-agentic-workflows-intro.md`). Do not remove any of the 5 existing checklist items — add the worked example as a new subsection rather than compressing existing content.

### Failure mode classification

**Learning barrier** — `concept-overload` reflects conceptual density and missing worked-example scaffolding, not an environment, tooling, or auth problem.

### Quantitative guardrail

- Current `overall_score`: 5.39/10
- Weakest rubric dimension(s): `active_learning` (3.9/10) and `checkpoint_quality` (0.0/10 — the page uses `## :white_check_mark: Checkpoint`, so this may be a rubric-detector artifact rather than a true missing checkpoint; verify the shared `CHECKPOINT_RE` pattern before assuming a real gap)
- Current learning KPI index: `(2.0 × 3.9 + 2.0 × 0.0 + 1.5 × 5.0) / 5.5 = 2.78/10`
- Why the proposed change should raise these scores: a worked example is a direct `active_learning` improvement (more guided hands-on practice, not just checklist items), and the prerequisite-skip framing improves `scaffolding` by making entry assumptions explicit. Neither change reduces content or lowers cognitive challenge for learners who need the full explanation — it only adds an optional fast path and a concrete example, so `overall_score` and the KPI index should not decrease.

### Acceptance criteria

- [ ] The shared curriculum quantitative assessment `overall_score` for `04-github-actions-intro.md` stays flat or improves after the change
- [ ] The step's learning KPI index `(2.0 × active_learning + 2.0 × checkpoint_quality + 1.5 × scaffolding) / 5.5` stays flat or improves versus the current 2.78/10 baseline
- [ ] All 5 existing checklist items and the existing checkpoint section are preserved unchanged
- [ ] A new worked example subsection and a prerequisite-skip framing sentence are added without removing any existing vocabulary or concept coverage

### Suggested owner

`copilot coding agent`
Related to #3447

> Generated by [🔬 Workshop Student Simulator](https://github.com/githubnext/gh-aw-workshop/actions/runs/35109821545) · copilot · auto · 164.9 AIC · ⌖ 7.67 AIC · ⊞ 15.2K · [◷](https://github.com/search?q=repo%3Agithubnext%2Fgh-aw-workshop+is%3Aissue+%22gh-aw-workflow-call-id%3A+githubnext%2Fgh-aw-workshop%2Fworkshop-student-simulator%22&type=issues)
> - [x] expires on Sep 17, 2026, 2:52 PM UTC

Start with 04-github-actions-intro.md and compare its structure with the already-familiar skip pattern in 05-agentic-workflows-intro.md. Add the prerequisite framing and a worked trigger-to-job-to-step example without changing the five checklist items or checkpoint. Run the shared curriculum quantitative assessment and verify the scores do not decline.

Written by the indexing model from the issue text.