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>

README.md

@@ -3,10 +3,11 @@
 A command-line client for the [Invoice Ninja](https://www.invoiceninja.com/)
 v5 API, built on [`invoiceninja-sdk`](https://crates.io/crates/invoiceninja-sdk).
 
-> **Early scaffold.** `clients`, `invoices`, `quotes`, `payments`, `products`,
-> and `expenses` are wired up so far, out of ~34 resources — see `CLAUDE.md`
-> for the pattern the rest follow. [`GOALS.md`](GOALS.md) covers what the
-> crate is for and what it is not.
+> **Early scaffold.** `clients`, `invoices`, `quotes`, `credits`, `payments`,
+> `products`, `expenses`, `projects`, `tasks`, `vendors`, and
+> `purchase-orders` are wired up so far, out of ~34 resources — see
+> `CLAUDE.md` for the pattern the rest follow. [`GOALS.md`](GOALS.md) covers
+> what the crate is for and what it is not.
 
 ## Development
 
@@ -48,7 +49,8 @@ table view's few key columns:
 invoiceninja-cli --output json clients get <id>
 ```
 
-Every other wired-up resource (`invoices`, `quotes`, `payments`, `products`,
-`expenses`) follows the same `list`/`get`/`create`/`update`/`delete` shape,
-e.g. `invoiceninja-cli invoices list --status active` or
-`invoiceninja-cli payments get <id>`.
+Every other wired-up resource (`invoices`, `quotes`, `credits`, `payments`,
+`products`, `expenses`, `projects`, `tasks`, `vendors`,
+`purchase-orders`) follows the same `list`/`get`/`create`/`update`/`delete`
+shape, e.g. `invoiceninja-cli invoices list --status active` or
+`invoiceninja-cli purchase-orders get <id>`.