Add Step 12 compile-first iteration guidance to reduce workflow-not-compiled dropoff

[Copilot]

Step 12 is the workshop’s largest dropout point, and the dominant failure mode is workflow-not-compiled. The simulator currently maps no workshop content to that step, so learners get no in-flow reminder that workflow edits must be recompiled before the next test run.

• Add the missing Step 12 node

  • Introduces workshop/12-test-and-iterate.md as the canonical iteration step.
  • Centers the learner loop on one change at a time: observe, edit, compile, re-run, compare.

• Make compile-before-test explicit

  • Adds a hard reminder that GitHub Actions executes the compiled .lock.yml, not the source .md.
  • Places gh aw compile in the critical path before every validation run, targeting the exact failure category driving attrition.

• Repair workshop flow and discoverability

  • Routes Step 9 forward to Step 12 instead of skipping directly to “Next steps”.
  • Adds Step 12 to workshop/README.md so curriculum and simulator-visible structure stay aligned.
  • Updates workshop/00-welcome.md to reflect the compile → test → iterate loop learners actually need.

Example of the new guidance added to the workshop:

> [!IMPORTANT]
> Compile after every workflow edit before you test it. GitHub Actions runs the
> compiled `.lock.yml` file, not the source `.md` file, so skipping `gh aw compile`
> means you are testing stale workflow logic.

```bash
gh aw compile