Clarify GitHub UI workflow compilation flow for Step 7

[Copilot]

The GitHub UI path implied that committing hello-agent.md was enough for the workflow to appear in Actions, but GitHub does not compile the source Markdown automatically. This update makes the UI-only path explicit: learners must use the Agentic Workflows agent in the Agents tab to generate and commit hello-agent.lock.yml before running the workflow.

• Step 7b — GitHub UI path

  • Reframes the outcome to include compiling the workflow via the Agentic Workflows agent.
  • Adds an explicit Agents tab compile step with a copy/paste prompt.
  • Updates the checkpoint to require hello-agent.lock.yml and review of the generated diff.

• Step 7 overview

  • Updates the path chooser to describe the UI path as a two-step flow: author in the web editor, then compile via the agent.
  • Removes the implication that the UI path can rely on later automatic compilation.

• Step 8 prerequisites and recovery

  • Requires both hello-agent.md and hello-agent.lock.yml on main before the first run.
  • Adds a clear recovery path when Hello Agent does not appear in Actions: return to Step 7b and compile via the Agentic Workflows agent.

Compile `.github/workflows/hello-agent.md` with `gh aw compile`.

If the compile succeeds, commit the generated `.github/workflows/hello-agent.lock.yml` file to `main` and show me the diff before I approve it.

If the compile fails, fix the workflow and show me the diff before you commit.