Add fast-path guidance for experienced users in Steps 4 and 5

[Copilot]

Advanced users were dropping in the GitHub Actions intro sequence because Steps 4–5 did not clearly signal a faster path to the hands-on agentic workflow content. This update makes the skip path explicit while preserving the beginner-oriented explanations.

• Step 4: experienced Actions user fast path

  • Added a visible > [!TIP] callout near the top of 04-github-actions-intro.md
  • Directs users already comfortable with Actions workflows and YAML to skip to Step 6
  • Keeps Step 5 positioned as optional conceptual background

• Step 5: experienced automation user fast path

  • Added a visible > [!TIP] callout near the top of 05-agentic-workflows-intro.md
  • Directs users familiar with LLM-based automation to review a short summary, complete Step 6, then jump to Step 7

• Step 5: concise delta from classic Actions

  • Added a Quick Summary box that focuses on the actual transition from classic Actions to gh-aw
  • Highlights:
    • YAML frontmatter remains
    • workflow logic moves from fixed jobs.steps scripts to a Markdown task brief
    • runtime behavior is constrained by declared permissions and safe outputs

> [!TIP]
> Already comfortable with GitHub Actions workflows and YAML? You can skip this step and go straight to [Step 6: Install the gh-aw CLI Extension](06-install-gh-aw.md).

## Quick Summary

> **Key differences from classic Actions:**
> - A `gh-aw` workflow still starts with YAML frontmatter fenced by `---`, usually with `name`, `on`, and `permissions`.
> - Below that frontmatter, you write a Markdown task brief instead of a fixed `jobs.steps` script.
> - The agent uses that brief plus the declared permissions and safe outputs to decide how to complete the work at runtime.