chore(release): migrate from Lerna to Changesets

[petermuessig]

Summary

Replaces Lerna with Changesets for versioning and publishing. The release flow now mirrors what's in use in ui5-community/ui5-ecosystem-showcase: contributors add a .changeset/*.md to their PR, merging to main opens/updates a Version Packages PR, and merging that PR publishes to npm.

What changed

File	Change
.changeset/config.json, .changeset/README.md	New — independent versioning, GitHub changelog, ignores private @ui5/openui5-snapshot-test and @ui5/typed-json-model
scripts/auto-changeset.mjs	New — derives .changeset/auto-<sha>.md files from conventional commits (scope -> packages, type -> bump). Idempotent; hand-written changesets always win
.github/workflows/release.yml	Rewritten to use changesets/action@v1.8.0 with npm OIDC trusted publishing + provenance, fallback to NPM_BOOTSTRAP_TOKEN
.github/workflows/changeset.yml	New — PR gate that fails when no changeset is present. Counts .changesets rather than .releases so empty changesets pass
package.json	Removed lerna, added @changesets/cli + @changesets/changelog-github. Replaced release:* scripts with changeset / version-packages / release-publish / changeset:auto / changeset:empty. build is now explicit per workspace (the test-packages have no build script)
CONTRIBUTING.md	Rewritten end-to-end: PR checklist, three-command picker for changesets, bump-derivation table, maintainer release flow. Legal / DCO / AI-generated-code sections preserved
lerna.json	Deleted
yarn.lock	Refreshed

New contributor flow

# In your feature branch:
yarn changeset:auto       # walk conventional commits and write .changeset/auto-*.md (recommended)
yarn changeset            # interactive picker, when auto-derived bump/summary is wrong
yarn changeset:empty      # docs/CI-only PRs that don't need a release entry

New release flow (no local lerna, no push-to-main, no npm creds)

1. Merge a PR with at least one changeset to main.
2. The Release workflow opens/updates a single Version Packages PR (versions bumped, CHANGELOGs regenerated, lockfile refreshed).
3. Merge that PR.
4. The same workflow publishes the new versions to npm with provenance and pushes the matching tags.

Verification

• yarn install — ok, lockfile updated cleanly
• yarn build — both @ui5/dts-generator and @ui5/ts-interface-generator compile (incl. api-extractor)
• yarn changeset status — exit 0 (no pending changesets, expected)
• yarn changeset version — No unreleased changesets found, exiting (expected)
• node scripts/auto-changeset.mjs --dry-run --verbose — range resolved against last chore(release): publish commit, 0 commits since
• yarn format:validate — clean

Not in this PR (intentional)

• Yarn classic stays for now. A pnpm migration is a logical follow-up (the showcase uses pnpm; pnpm install --lockfile-only is a cleaner fit for the version-packages step than yarn classic), but it's a separate diff worth reviewing on its own.
• The informational Package "@ui5/openui5-snapshot-test" must depend on the current version of "@ui5/dts-generator" warnings from changeset status are harmless under yarn classic's link: syntax. They go away with the pnpm migration.

Required follow-up before merging

• Confirm secrets.NPM_BOOTSTRAP_TOKEN is configured on UI5/typescript (or that npm OIDC trusted publishing is set up for @ui5/dts-generator and @ui5/ts-interface-generator).
• After merging, the first push to main will open an empty Version Packages PR — that's expected.

[petermuessig]

@codeworrior @akudev - here is the change to get rid of the local release steps and make this part of GH actions by using changesets - I can also explain to you how this new release flow will work.

[Copilot]

Pull request overview

Migrates the monorepo release tooling from Lerna to Changesets, introducing an automated release PR + publish workflow and a PR gate to require changesets for all incoming changes.

Changes:

• Replace Lerna-based versioning/publishing scripts with Changesets CLI workflows and scripts.
• Add scripts/auto-changeset.mjs to derive .changeset/*.md entries from conventional commits.
• Add GitHub Actions workflows for release automation and PR-level changeset enforcement; update contributor documentation accordingly.

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
scripts/auto-changeset.mjs	New helper CLI to generate changesets from conventional commits.
package.json	Removes Lerna scripts/dependency; adds Changesets scripts and deps.
lerna.json	Removed (Lerna no longer used).
CONTRIBUTING.md	Rewrites contribution + release guidance to the new Changesets flow.
.github/workflows/release.yml	Reworks release automation to use changesets/action.
.github/workflows/changeset.yml	New PR workflow to require a changeset (including empty changesets).
.changeset/config.json	Adds Changesets configuration (independent versioning, GitHub changelog, ignores).
.changeset/README.md	Adds repo-specific Changesets documentation.
.changeset/fresh-cobras-send.md	Adds an empty changeset entry.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[akudev]

Edge case, but skip-if-covered drops uncovered packages:

const overlap = pkgNames.filter((n) => covered.has(n));
if (overlap.length > 0) {
    return { skip: `already covered (${overlap.join(", ")})` };
}

If a hand-written changeset already covers @ui5/dts-generator and a subsequent commit touches both @ui5/dts-generator and @ui5/ts-interface-generator, the entire commit is skipped — missing the ts-interface-generator change. The fix would be to filter out the covered packages and proceed with the uncovered ones:

const uncovered = pkgNames.filter((n) => !covered.has(n));
if (uncovered.length === 0) {
    return { skip: `already covered (${pkgNames.join(", ")})` };
}
// continue with uncovered instead of pkgNames