Add Codespace bridge callout to Step 6 (Install gh-aw) for non-terminal users

[Copilot]

~26% of simulated students (non-CLI users: web UI, Copilot app, cloud agent) hit Step 6 with no terminal and no guidance, producing a 0% completion rate at the workshop's highest-dropout step.

Changes

• workshop/06-install-gh-aw.md — Inserts an [!IMPORTANT] callout between the prerequisites and the Steps section, directing non-terminal users to open a Codespace as a zero-install path, linking to 02a-setup-codespace.md for first-time setup, and noting the extension persists for all subsequent steps once installed:

> [!IMPORTANT]
> **No local terminal?** If you are using the GitHub web UI, the Copilot app, or a cloud coding agent,
> open a **Codespace** on your practice repository — it comes with `gh` pre-installed and authenticated.
> Haven't set one up yet? Follow [Adventure A: Set Up a Codespace](02a-setup-codespace.md) first.
> Once `gh extension install github/gh-aw` succeeds inside the Codespace, the extension is available
> for all subsequent steps in that environment.