Refresh Docs for Stack CLI 0.5.2 and runtime examples

[konojunya]

Background

The public Docs sources need to describe the simplified built-in example gallery, point readers to the live product and four localized guides, and track the published Stack CLI v0.5.2 release. Contributor-only source ownership and synchronization details were obscuring the reader-focused README.

Implementation Approach

• lead the README with what Stack provides, how to start, and where the documentation is published
• move source ownership, generation, release synchronization, and consumer freshness guidance into CONTRIBUTING.md
• describe the four built-in example cards and the image dialog controls in every locale without requiring provider icon packs
• pin @stack-sh/engine 0.8.0 exactly for development compatibility checks
• synchronize the stable CLI release lock to v0.5.2 at commit f6220c5c24bc515574ea6f6dd6d55af00d35f89d and regenerate all derived content

Testing Approach

• Node.js 22.14.0: npm test
• Node.js 22.14.0: npm run check
• Node.js 22.14.0: npm run machine:check
• Node.js 22.14.0: npm run release:check
• verify the published aarch64 macOS archive with GitHub artifact attestations
• run npm run smoke against the verified v0.5.2 binary, including the generated skill and all four localized first-render walkthroughs

Review Perspectives / Points of Concern

• Published immutable machine v1 resources are unchanged; Engine 0.8.0 is only the development conformance dependency.
• Consumer freshness is expected to report v0.5.1 until CLI, Web, and production update their reviewed Docs pins after this provider PR merges.