[plan] docs: Surface --engine claude in quick-start and document silent Copilot default

[github-actions]

Objective

Update docs/src/content/docs/setup/quick-start.mdx to show --engine claude in the primary workflow add example and explicitly state that omitting engine: defaults to Copilot.

Context

From documentation review discussion #43544 (Priority 2):

Silent Copilot default. Copilot is the default engine; 35 example workflows omit engine: entirely and resolve to Copilot. --engine claude exists (cli.md:142) but is not surfaced in the quick-start add flow.

A Claude user following the quick-start sees gh aw add without --engine claude and may not realize their workflow will run on Copilot by default.

Files to Modify

• docs/src/content/docs/setup/quick-start.mdx
  • In the gh aw add example step, show --engine claude as a variant or alongside the Copilot example
  • Add a callout stating: omitting engine: defaults to Copilot; pass --engine claude (or set engine: claude in frontmatter) to use Claude

Acceptance Criteria

• The quick-start gh aw add section shows --engine claude as an explicit option
• A note explains the default engine is Copilot when engine: is omitted
• Claude users can follow the quick-start without accidentally defaulting to Copilot

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• expires on Jul 7, 2026, 7:22 AM UTC-08:00