feat: add --screenshot option to slice add-variation and slice edit-variation

[angeloashmore]

Resolves: #95

Description

Add a --screenshot option to slice add-variation and slice edit-variation that accepts either a local file path or a URL. The image is uploaded to S3 via the ACL provider, and the resulting imgix URL is stored in the variation's imageUrl field.

Checklist

• A comprehensive Linear ticket, providing sufficient context and details to facilitate the review of the PR, is linked to the PR.
• 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

# With a URL
prismic slice edit-variation default --from-slice my_slice --screenshot https://example.com/image.png

# With a local file
prismic slice add-variation "With Image" --to my_slice --screenshot ./screenshot.png

---

Note

Medium Risk
Adds a new networked file-download + S3 upload flow (via ACL provider) to CLI commands, which can fail due to file type/protocol handling and external service interactions. Core slice update paths are unchanged but now conditionally include imageUrl updates.

Overview
Adds a new --screenshot/-s option to slice add-variation and slice edit-variation to set a variation’s imageUrl by uploading an image from a local path or HTTP(S) URL.

Introduces screenshot upload support in the custom-types client via the ACL provider (typed response validation, MIME-type allowlist, deterministic keying via MD5, and imgix URL generation) plus a new readURLFile() helper to load files from file:/http(s): URLs. Tests are extended to cover both URL-based and local-file screenshots for both commands.

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