Add consolidated React Router agent skill

[brookslybrand]

Summary

• add the React Router agent skill under .agents/skills/react-router
• consolidate mode-specific guidance into a single user-facing skill
• add thin references for Framework, Data, Declarative, RSC Data, and RSC Framework modes
• point agents toward local package docs via react-router/docs/... when available

Notes

This follows the direction from discussion #15099 to move React Router agent skills into this repo and keep the skill relatively thin, with deeper documentation coming from package-published markdown docs.

[github-actions]

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Preview Build Available

Preview builds have been created for this PR. You can install react-router using:

pnpm install "remix-run/react-router#preview/pr-15158&path:packages/react-router"

And/or install other packages via:

pnpm install "remix-run/react-router#preview/pr-15158&path:packages/react-router-dev"
pnpm install "remix-run/react-router#preview/pr-15158&path:packages/react-router-express"
pnpm install "remix-run/react-router#preview/pr-15158&path:packages/react-router-node"
pnpm install "remix-run/react-router#preview/pr-15158&path:packages/react-router-serve"

These preview builds will be updated automatically as you push new commits.