Build in Accounting Concepts

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As I've learned more about accounting, I can see how it would help add some structure and solve some problems around tracking actual expenses vs activity in the repository. At very least, I think the `Accounts` type should understand some basic things intrinsically, like assets, liabilities, and equity. That way tools that are building on top of it can have a correct notion of credits/debits and zero-based accounting/budgeting.

I'm not sure how to marry together the more formal accounting with the concept of budgeting yet. They're surprisingly different. But I think I'll essentially keep them separate, and have the budget and each child budget essentially be automatic expense accounts. The budget is essentially the target or max spending for each expense account dure a period.

This will be a major breaking change.

No files or tests are named. Start by locating the Accounts type and the existing budget, child-budget, and expense-account concepts, then document how they currently represent expenses and activity. The issue does not define a settled accounting model or concrete completion criteria, so the design and breaking-change scope must be resolved before implementation.

Written by the indexing model from the issue text.