feat(skills-next): add react-router-framework SDK references

[evanpurkhiser]

Direct LLM port of the existing react-router-framework SDK skill into the skills-next per-SDK reference layout under skills-next/references/sdks/react-router-framework/. This content has NOT been reviewed at all — it is a machine-generated port of the existing SDK skill, and every file should be treated as unverified.

[sentry]

Bug: The documentation contains relative links to other SDKs, like ../react/index.md, that do not exist. This breaks navigation between reference documents.
_{Severity: MEDIUM}

Suggested Fix

The original skill used skill name references like sentry-react-sdk. The fix should either restore a similar reference-based system that the skills-next infrastructure can resolve or create the missing SDK documentation files at the expected relative paths to make the links functional.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: skills-next/references/sdks/react-router-framework/index.md#L380-L384

Potential issue: The ported documentation for the React Router framework SDK replaces
functional skill name references (e.g., `sentry-react-sdk`) with relative file path
links (e.g., `../react/index.md`). These links are broken because the target directories
and files do not exist within the `skills-next/references/sdks/` directory structure.
This is a functional regression, as an LLM agent or user attempting to follow these
navigational links for cross-SDK guidance will fail, breaking expected handoffs between
different SDK references.

Also affects:

• skills-next/references/sdks/react-router-framework/index.md:6~6
• skills-next/references/sdks/react-router-framework/react-router-framework-features.md:96~97

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[evanpurkhiser]

Merging these ahead of review to get the ball rolling on the new skills layout. @cleptric will follow up with SDK teams to finesse the content of the SDK references.