[side-quest] Extract gh-aw install troubleshooting into an optional side quest

[Copilot]

Step 6 contained a large troubleshooting appendix that interrupted the main install flow despite being optional reference material. This change extracts that content into a dedicated side quest and leaves Step 6 focused on the primary install path.

• Step 6 now routes to optional troubleshooting

  • Replaced the long inlined troubleshooting block in workshop/06-install-gh-aw.md with a short summary.
  • Added a > [!TIP] callout linking to the new side quest.

• New side quest: install troubleshooting reference

  • Added workshop/side-quest-install-troubleshooting.md.
  • Includes copy-paste fixes for four scenarios:
    • Not authenticated (HTTP 401)
    • Corporate proxy environment
    • GitHub Enterprise Server hostname usage
    • Locked-down network install fallback
  • Ends with a direct return link back to Step 6.

• Workshop index updated

  • Added the new side quest to workshop/README.md under Optional Side Quests, marked as branching from Step 6.

Example of the new Step 6 routing pattern:

## Troubleshooting

Most installs work on the first try after `gh auth status` and `gh extension install github/gh-aw`.
If you hit auth, proxy, GHES hostname, or locked-down-network errors, use the optional troubleshooting side quest.

> [!TIP]
> Need copy-paste fixes? Open [Side Quest: Install `gh-aw` Troubleshooting](side-quest-install-troubleshooting.md), then come back here.