skill(next-cache-components-adoption): clarify prereqs (#95082)
### What
Driven by three friction logs from agents running
`next-cache-components-adoption` against starting points the skill
silently assumed.
- Restructure `## requires` as a labeled checklist: **App Router
project**, **Next.js 16.3 or later**, **No incompatible config keys**.
Each item names what's checked, what the failure mode is, and where to
resolve it.
- Hoist the **No green baseline before the flag** note out of the prereq
pile into a separate `### notes` block (alongside **Offline docs**), so
a reader scanning prereqs doesn't conflate it with a hard requirement.
- Tighten the codemod block: explain `@canary`'s necessity in one line
so an agent hitting `Invalid transform choice` from `@latest` knows the
workaround.
### Why
Three friction logs each hit a different version of the same gap:
- **react.dev migration**: agent ran codemod with `@latest` (16.2.9) →
`Invalid transform choice` → applied transform by hand.
- **Marketing-site case**: agent built the spec with `revalidate`
exports, codemod left them in, `cacheComponents: true` rejected the
build.
- **E-commerce case**: `experimental.dynamicIO`/`useCache` left in
`next.config` → fatal config error.
In each case the skill *technically* said what to do but it was buried.
### Related
The migration-guide changes that were originally in this PR moved to
[#94997](https://github.com/vercel/next.js/pull/94997).
### Type of Change
- [x] Documentation (adds or changes documentation)
<!-- NEXT_JS_LLM_PR -->
