docs: clarify first-party Skills workflows (#96495)
## Summary
- clarify how runtime foundations, interactive workflows, and unattended
loops fit into the AI agents and Instant Navigation guides
- harden the Cache Components and Partial Prefetching Skills around
capability checks, trustworthy `instant()` tests, route scope, and
real-app verification failures
- add migration-guide comments to temporary Partial Prefetching route
config and update `remove-partial-prefetch` to remove only generated
comments while preserving runtime-prefetch TODOs and user comments
This keeps the experimental Partial Prefetching optimizer in #96471
separate.
## Testing
- `pnpm --filter @next/codemod build`
- `pnpm --filter @next/codemod test --
transforms/__tests__/remove-partial-prefetch.test.js --runInBand`
