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.