Redirects: resolve cross-repo to: in isolated CI

[reakaleek]

Why

• Isolated build --strict on content PRs did not resolve cross-repo redirects.yml to: values against the published link index.
• A GitHub blob path such as kibana://docs/reference/advanced-settings.md passed required CI and broke assembler-preview on unrelated docs-builder PRs after merge.

What

• Isolated builds now resolve :// redirect targets with the same TryResolve path assembler uses.
• Writer docs state that the path is relative to the target repo docset.yml directory, and that diff validate only checks keys.

Notes

• Same-repo to: values still use the local file check. https:// targets and --skip-cross-links stay unchanged.
• This does not close [Feature Request]: Validate redirects.yml inputs #1538. That issue covers input lint on redirect keys, not to: index membership.

Made with Cursor

[Mpdreamz]

Thanks for plugging that gap!