fix website release history loading

[siddarthvader]

What changed

• resolve the repository root by walking upward from the website build working directory
• require both the root changelog and Changesets configuration before loading release data
• fail the website build when the changelog contains no releases instead of silently rendering an empty history

Why

The changes loader derived the repository root from import.meta.url. Astro and Vite relocate that module during the production build, so the resulting path pointed outside the repository. The missing CHANGELOG.md and .changeset directories were treated as empty data, producing a successful deployment with an empty Changes page.

Impact

The statically generated Changes page now includes the published SDK history. This is a website-only fix and does not require a package changeset.

Validation

• Prettier check for website/src/data/changes.ts
• loader verification from the website build working directory
• Astro static build into a temporary output directory
• generated HTML contains 0 pending and releases v0.5.0 through v0.1.0
• git diff --check