Add credits, projects, tasks, vendors, and purchase orders

Five more resources following clients.rs's template exactly, same as
the previous batch. credits.rs is otherwise identical to quotes.rs
since Credit is also a type alias for Invoice in the SDK (same table,
distinguished by entity_type upstream).

Verified against a live local server: full create/list/get/update/
delete round trip for all five.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CLAUDE.md

@@ -20,9 +20,10 @@ design call, the same way its counterpart in the SDK works. If a change
 would contradict a stated goal or settle an open question, update
 `GOALS.md` in the same change.
 
-**Early scaffold.** Six resources are wired up (`clients`, `invoices`,
-`quotes`, `payments`, `products`, `expenses`) out of ~34; `clients` was the
-template the other five followed. See "Adding a resource" below before
+**Early scaffold.** Eleven resources are wired up (`clients`, `invoices`,
+`quotes`, `credits`, `payments`, `products`, `expenses`, `projects`,
+`tasks`, `vendors`, `purchase_orders`) out of ~34; `clients` was the
+template the other ten followed. See "Adding a resource" below before
 writing the next one.
 
 ## Development environment