build(readme): sync only shared blocks so packages keep their own READMEs

[apresmoi]

Summary

• replace the wholesale root-README copy in sync-package-readmes.mjs with a partial sync: only regions wrapped in <!-- polycss:shared:<name>:start/end --> are mirrored from the root README, and everything between them is package-specific
• mark four shared blocks in the root README — links, packages, showcase, license — and adopt them in the core, polycss, react, and vue READMEs
• give those four packages real, hand-written READMEs: core documented the wrong package (it told readers to npm install @layoutit/polycss and showed JSX), and vue shipped React TSX examples
• a package opts into a block simply by containing its markers, so fonts and morph need no exemption list and are left untouched
• add pnpm check:readmes (--check) and run it in CI, so a shared block edited in the wrong place fails the build instead of drifting
• document the contract in AGENTS.md and add a PR-checklist line

Previously prepack copied the root README over all four packages on every npm pack / npm publish, so a package-specific README could not reach npm — the file reviewed in a PR was not the file published. Now what is committed is what ships.

Validation

• pnpm test
• pnpm build:packages
• pnpm check:readmes
• npm pack --dry-run in packages/core: prepack reports 0 READMEs updated, and the tarball carries core's own 5.9 kB README rather than the 9.9 kB root copy
• drift check verified by hand: mutating a shared block in a package README fails --check with a non-zero exit, and passes again once restored