[Parent] Engine documentation updates

[github-actions]

This parent tracks the coordinated updates to engine documentation and onboarding flow. Related issues:

• [plan] Document engine selection in Quick Start interactive flow #12035 [plan] Document engine selection in Quick Start interactive flow
• [plan] Add 'Getting Started with Claude' example to engines.md #12034 [plan] Add 'Getting Started with Claude' example to engines.md
• [plan] Update README.md engine ordering to show engines as equals #12033 [plan] Update README.md engine ordering to show engines as equals
• [plan] Add engine comparison table to engines.md #12032 [plan] Add engine comparison table to engines.md
• [plan] Remove 'experimental' label from Claude/Codex engines documentation #12031 [plan] Remove 'experimental' label from Claude/Codex engines documentation

These items are tightly coupled documentation changes that should ship together to keep engine guidance consistent.

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