feat(sveltekit): support SvelteKit 3 and @prismicio/svelte v3

[angeloashmore]

Resolves:

Description

This PR adds support for SvelteKit 3. SvelteKit 3 projects get @prismicio/svelte v3 and code that builds on SvelteKit 3.

It keeps SvelteKit 2 projects working as before, with @prismicio/svelte v2.

Merge this PR only after @prismicio/svelte v3 is on npm.

Checklist

• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

Preview

How to QA ¹

After v3 is published, run prismic init in a new sv create project, then run npm run build. Open the slice simulator in the Page Builder and make sure that it shows your slices. Then do the same in a SvelteKit 2 project and make sure that it installs @prismicio/svelte v2.

🤖 Generated with Claude Code

https://claude.ai/code/session_01L53y5wowGHwtT4aZJ3UsHY

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Note

Medium Risk
Framework detection and generated project scaffolding changed across two SvelteKit majors; mistakes could break init/setup for new or legacy Kit projects until caught by e2e tests.

Overview
Adds SvelteKit 3–compatible codegen and keeps SvelteKit 2 on a separate adapter path so prismic init / gen setup produce projects that build under current sv create.

SvelteKit 3 (default adapter): Generated code switches to #lib/... imports with file extensions, reads PUBLIC_PRISMIC_ENVIRONMENT from $app/env/public, and creates or documents src/env via defineEnvVars. The slice simulator uses @prismicio/svelte v3’s snippet API instead of let:slices. Setup instructions can include an env-var step when an existing src/env omits PUBLIC_PRISMIC_ENVIRONMENT.

SvelteKit 2: getAdapter() picks SvelteKit2Adapter when installed @sveltejs/kit is major < 3. That adapter extends the main one but pins @prismicio/svelte ^2 and reuses templates transformed for $lib, $env/dynamic/public, and slot-based SliceSimulator.

Shared helpers (findSourceFile, exported deleteStarterHomePage) support both paths. Tests and preview evals cover SK2/SK3, env handling, and #lib vs $lib page generation; AGENTS.md documents version-specific adapter files.

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