Prototype store info diagnostic event sink

[dmerand]

What

Add a private, dependency-free @shopify/diagnostics leaf and a typed synchronous diagnostic side channel to store:info. The CLI adapter connects those events to the existing debugMessageSink, so current output and result behavior remain unchanged.

Why

Shopify CLI is moving toward data-only finite execution contracts that can serve CLI, SDK, MCP, and other adapters without depending on terminal rendering. This proof of concept demonstrates the compatibility seam between renderer-first execution and future explicit result/event adapters without claiming a universal event framework.

How

• Keep diagnostics data-only and synchronous; observers run in registration order and observer failures do not affect command results.
• Keep domain events owned by store:info, with typed variants and safe error projections.
• Preserve the existing JSON wire shape, stdout/stderr separation, debug filtering, timestamps, color handling, test log collection, fallback/result/error behavior, and non-interactive behavior through the existing CLI output adapter.

Async delivery, stream/progress protocols, universal result/error/event envelopes, launcher or BaseCommand changes, infrastructure logging, and broader command migration are out of scope.

Verification and limitations

• 55 focused tests pass.
• @shopify/diagnostics build, type-check, and lint pass.
• cli-kit type-check, changed-file ESLint, and git diff --check pass.
• Full store type-check remains blocked by the pre-existing missing @shopify/organizations declarations; no errors were reported in the changed diagnostics or store-info code.

Post-release steps: None.