fix: add import attribute to release-prefixes.json import

[lukemelia]

Problem

The boxel-cli publish workflow has failed on every push to main since ~Jun 26 at its "Compute release" step:

TypeError [ERR_IMPORT_ATTRIBUTE_MISSING]: Module ".../scripts/release-prefixes.json" needs an import attribute of "type: json"

CI runs Node 24, which rejects bare JSON module imports. Because the publish pipeline is broken, no new versions of @cardstack/boxel-cli have shipped to npm since Jun 19 — including the atomic-upload positional-mapping fix (#5413) that the boxel-skills "Sync to Workspace" action needs.

Fix

Add the required import attribute to the one bare JSON import in packages/boxel-cli/scripts/:

import bumpByPrefixJson from './release-prefixes.json' with { type: 'json' };

Verification

• Reproduced the crash locally on Node 24.17.0 (same version as CI) with the unfixed script; the fixed script runs cleanly and emits its release JSON.
• pnpm exec vitest run tests/scripts/compute-release.test.ts — 30/30 pass.
• eslint + prettier clean.

Note: scripts/compute-release.ts is not part of the npm or plugin bump surfaces, so this PR itself computes a no-op release; its purpose is to unbreak the workflow for subsequent merges and the promote path.

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