Add invoices, quotes, payments, products, and expenses

Five more resources following clients.rs's template exactly: the
same list/get/create/update/delete shape, including the
fetch-merge-write pattern for update. quotes.rs is otherwise
identical to invoices.rs since Quote is 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 six resources, confirming update preserves
fields the caller didn't mention.

Also adds CHANGELOG.md (Keep a Changelog format, matching the SDK's)
and updates README/CLAUDE.md's resource counts.

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

src/cli.rs

@@ -45,13 +45,23 @@ pub enum OutputFormat {
     Json,
 }
 
-/// One variant per resource. Only `clients` is wired up so far; it's the
-/// template for adding the rest of the SDK's ~34 resources the same way —
-/// see CLAUDE.md.
+/// One variant per resource. Six are wired up so far (`clients` was the
+/// template for the rest); see CLAUDE.md's "Adding a resource" for the
+/// remaining ~28.
 #[derive(Subcommand)]
 pub enum Command {
     /// Manage clients
     Clients(ResourceArgs),
+    /// Manage invoices
+    Invoices(ResourceArgs),
+    /// Manage quotes
+    Quotes(ResourceArgs),
+    /// Manage payments
+    Payments(ResourceArgs),
+    /// Manage products
+    Products(ResourceArgs),
+    /// Manage expenses
+    Expenses(ResourceArgs),
 }
 
 #[derive(Args)]