chore(schematics): document the scrollbar-deprecated-path migration

[lskramarov]

Summary

packages/schematics/src/migrations/scrollbar-deprecated-path/ was the only migration of 20 without a README.md, so it shipped to consumers undocumented while all its siblings ship a guide.

The missing file turned out to be more than a documentation gap. packages/schematics/scripts/copy-meta-to-dist.js copies each migration's README into the published package, its loop is sequential and rethrows, and the final rejection was swallowed — so the copy step aborted at the first missing file and the build still exited 0. Verified locally: with the README absent, dist/components/schematics/migrations/v20-upgrade/index.js and dist/components/schematics/utils/messages.js were never copied, yet yarn run build:schematics reported success. rimraf packages/schematics/dist does not clear dist/components, so a stale local build masks the gap while CI publishes the truncated package.

List of notable changes:

• added README.md for the scrollbar-deprecated-path migration, following the structure of the existing ones — what the schematic rewrites, which specifiers the quote-anchored match deliberately skips, the fix default, and a before/after example.
• documented the silent part of the entry-point split: @koobiq/components/scrollbar and @koobiq/components/scrollbar/deprecated share only KbqScrollbar and KbqScrollbarOptions, so most unmigrated imports fail with TS2305, while <kbq-scrollbar> keeps compiling and rendering against the new component and the [kbq-scrollbar] attribute form of the selector quietly matches nothing.
• fixed copy-meta-to-dist.js so a copy failure fails the build: the catch now sets process.exitCode = 1, failures are collected instead of aborting the loop at the first one, and a missing README.md / schema.json is reported as No README.md for the <migration> migration instead of a raw ENOENT path.

Verified with yarn run build:schematics on a cleared dist/components (exit 0, all 20 migrations complete), a negative run with alert-signals/README.md and v20-upgrade/schema.json removed (exit 1, both reported by name, everything else still copied), and npx jest packages/schematics (21 suites, 275 tests).

What should reviewers focus on?

• Whether the README's account of the behaviour change is what consumers should be told — in particular the claim that repointing the import preserves current rendering exactly, and that adopting the new scrollbar is a separate manual step.
• Whether README.md and schema.json should be hard requirements for every migration directory. All 20 provide both today, so the new check is green, but it does mean a future migration cannot land without its README.

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Pull request overview

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Suppressed comments (1)

packages/schematics/scripts/copy-meta-to-dist.js:97

• The aggregated failure summary says files are "missing", but failures also includes non-missing copy failures from copyFileWrapper (e.g. permission/IO errors). This can mislead when debugging CI vs local issues; consider wording that covers both missing files and copy errors.

    if (failures.length > 0) {
        throw new Error([`${failures.length} file(s) missing from the package:`, ...failures].join('\n  '));
    }