fix: tell the agent the receipt policy the books actually use

The server's instructions hardcoded '$25.00'; they now state the
configured threshold, and a zero threshold reads 'every entry needs at
least one receipt'.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MtYEaR6M6Ybdhm6X98BWNu

crates/ledger-mcp/src/server.rs

@@ -4051,7 +4051,11 @@ impl ServerHandler for BooksServer {
                 .enable_resources()
                 .build(),
         )
-        .with_instructions(INSTRUCTIONS)
+        // Tell the agent the policy these books actually use.
+        .with_instructions(INSTRUCTIONS.replace(
+            "any entry moving more than $25.00 in debits needs at least one receipt",
+            &self.policy.rule(),
+        ))
         .with_server_info(Implementation::new(
             "wanderbooks",
             env!("CARGO_PKG_VERSION"),