Docs Mergefield Sidebar Add-on

I already have a working Apps Script that shows a sidebar in an individual Google Doc and lets me click merge-fields into the page. The missing piece is packaging this into a true Google Docs add-on that appears in every document I open (private use only, no Marketplace listing).

What I need fixed or finished:
• Convert my existing script into a Google Workspace add-on for Docs so it loads automatically from Extensions → Add-ons.
• Populate the sidebar with all available merge-fields by syncing to a single Google Sheet (the Sheet is the source of truth; add-on should pull the latest list on open).
• When I click a field in the sidebar it must insert that placeholder at the current cursor position in the document.
• Keep OAuth scopes to the minimum: read the Sheet, modify the active Doc, display sidebar.
• Provide clear instructions (or a short loom / README) so I can deploy and authorise the add-on in my own Workspace account without publishing it publicly.

I’m happy to share the current code and Sheet structure right away, and I can test quickly inside several documents to confirm it behaves as expected.