Mark POS .observe extension targets as @deprecated

[vctrchu]

Summary

Adds @deprecated and @private JSDoc annotations to all 4 POS observe extension targets, pointing developers to pos.app.ready.data + shopify.addEventListener() as the replacement.

Targets deprecated

• pos.transaction-complete.event.observe → addEventListener('transaction_complete', cb)
• pos.cash-tracking-session-start.event.observe → addEventListener('cash_tracking_session_start', cb)
• pos.cash-tracking-session-complete.event.observe → addEventListener('cash_tracking_session_complete', cb)
• pos.cart-update.event.observe → shopify.cart.current.subscribe() on render targets, or addEventListener('cart_update', cb) on data target

Notes

• Annotations only — no runtime behavior changes
• @private excludes these from generated shopify.dev documentation
• Existing extensions using these targets continue to work unchanged

Resolves: https://github.com/shop/issues-retail/issues/26066
Epic: https://github.com/shop/issues-retail/issues/26019

Test plan

• yarn build passes
• CI passes (annotations only, no runtime changes)

[github-actions]

🚨🚨🚨 Docs migration in progress 🚨🚨🚨

We are actively migrating UI extension reference docs to MDX in the areas/platforms/shopify-dev zone of the monorepo. This impacts docs for the following surfaces:

• Admin UI extensions
• App Home
• Checkout UI extensions
• Customer account UI extensions
• POS UI extensions

During this migration, please be aware of the following:

.doc.ts files are being deprecated. Changes to .doc.ts files in this repo will not be reflected in the new MDX-based docs. If you need to update docs for a reference that has already been migrated, make your changes directly in the areas/platforms/shopify-dev zone of the monorepo instead.

Doc comments in .ts source files (the comment blocks above types and functions) are also affected. Generating docs from these comments currently requires a newer version of the @shopify/generate-docs library that isn't yet available. Updates to doc comments may not produce the expected output until the migration is complete.

Examples that previously lived in this repo are being moved to the areas/platforms/shopify-dev zone of the monorepo and should be authored there going forward.

What should I do?

• If your PR includes changes to .doc.ts files, doc comments, or examples, please reach out to us in #devtools-proj-templated-refs so we can help ensure your updates are captured correctly.
• If your PR is limited to source code changes (non-docs), you can ignore this notice.

Thanks for your patience while we complete the migration! 🙏