Recommend a no-cache rule for GitHub embeds on BYO domains

[Zach Dunn (zachdunn)]

Summary

Direction for #592: instead of building a platform-proxied embed twin for BYO hosts, recommend customers put the same badge-style cache rule embed.uploads.sh uses onto their own custom domain.

• Verify pipeline: the public-URL probe now records the response's Cache-Control, and a new recommended-only embed-cache check warns when it lacks no-store/no-cache — with the exact Transform Rule value in the hint. Emitted only when the domain actually answered, so it never stacks noise on an unreachable-domain failure, and it never gates ok.
• Settings form: embed-cache is the one check that doesn't block saving. If it's the only warning, the save proceeds and the tip renders on the lane card afterward; alongside real failures it renders in body color, not error red.
• Docs: /docs/byo-bucket gains an "Optional but recommended" setup step with the Transform Rule recipe and the edge-caching trade-off (plus the second-domain escape hatch for high-traffic buckets); the "What's different" GitHub-embeds bullet now says embeds work with the rule rather than being unsupported. The in-form setup help mirrors the step.

Testing

• New verify tests: warns on cacheable headers, passes on no-store/no-cache, omitted when the domain is unreachable
• Full suite passing (5165), typecheck clean

Closes #592.

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