[plan] docs: Add engine-choice guidance and document default Claude model

[github-actions]

Objective

Add a "which engine should I choose?" paragraph to the docs and document the default Claude model/version used when none is specified.

Context

From documentation review discussion #43544 (Priority 3 minor items):

• Copilot-first nudge "start there — no extra account setup" with no why-Claude-vs-Copilot guidance (quick-start.mdx:29)
• No default Claude model/version documented (architecture.mdx:279,286)

Currently the only guidance is a hint to "start with Copilot — no extra setup", with no explanation of trade-offs or when to pick Claude.

Files to Modify

• docs/src/content/docs/setup/quick-start.mdx — add a short engine-choice callout/paragraph near line 29 explaining Copilot vs Claude trade-offs (billing model, subscription vs API key, etc.)
• docs/src/content/docs/introduction/architecture.mdx — at lines ~279/286 where Claude engine config is described, document what the default model/version is when model: is omitted in the workflow frontmatter

Acceptance Criteria

• quick-start has a brief "which engine to choose" note covering key trade-offs (billing, setup friction, model capabilities)
• architecture.mdx (or a linked reference page) documents the default Claude model version used when none is specified in the workflow

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