Refresh fork from changesets action v1.9.0

[freben]

Refreshes the ancient fork baseline to upstream changesets/action v1.9.0 while retaining only the versionBranch input required by backstage/community-plugins. Removes fork-specific automatic release workflows and documents that backstage/backstage uses the official action directly.

Release preparation

• Continues the fork's existing v2 release lineage as the breaking v3.0.0 release rather than resetting it to upstream's v1.9.0 package version.
• Documents the manual build, verification, release, and immutable consumer-pin process.
• Allows the release commit to be created when dist is already current on main.
• Pushes the immutable v3.0.0 tag and moving v3 branch atomically, with force scoped only to the moving branch.
• Adds regression coverage for the release commit and ref-update contract.

After this PR is merged, v3.0.0 can be published by following the new Releasing this fork section in the README. Merging alone does not publish the release or update backstage/community-plugins.

Verification

• pnpm format
• pnpm typecheck
• pnpm exec vitest --run --testTimeout=30000 (14 tests)
• pnpm build
• git diff --check
• End-to-end smoke test against the exact PR head (30b512d) using a nested Changesets workspace, custom versionBranch, legacy GITHUB_TOKEN environment input, workspace-scoped file verification, generated PR-body verification, and generated-commit DCO verification.