docs: make the matcher snippets copy-pasteable

Review follow-ups on the P.tag migration:

- `docs/reference/error-model.md` and the `client.call` block in
  `docs/how-to/use-request-reply.md` use `P.tag(...)` in snippets that
  already lead with an import block, so declare `P`.
- `packages/client/README.md`: `./contract` gains its `.js` extension
  (the repo targets Node ESM), and the cleanup line uses
  `client.close().get()` — `close()` is `AsyncResult<void, never>`, so a
  bare `await` resolves to a `Result` and swallows a close defect.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 2 people

docs/how-to/use-request-reply.md

@@ -43,6 +43,8 @@ calculate: ({ payload }) =>
 ## Call it
 
 ```typescript
+import { P } from "unthrown";
+
 const result = await client.call("calculate", { a: 2, b: 3 }, { timeoutMs: 5_000 });
 
 result.match({

docs/reference/error-model.md

@@ -124,6 +124,7 @@ These are unexpected, so they surface through the **defect** channel, never as a
 
 ```typescript
 import { TechnicalError } from "@amqp-contract/core";
+import { P } from "unthrown";
 
 result.match({
   ok: () => {},

packages/client/README.md

@@ -21,7 +21,7 @@ pnpm add @amqp-contract/client
 ```typescript
 import { TypedAmqpClient } from "@amqp-contract/client";
 import { P } from "unthrown";
-import { contract } from "./contract";
+import { contract } from "./contract.js";
 
 // Create client from contract (automatically connects and waits for connection)
 const client = await TypedAmqpClient.create({
@@ -47,7 +47,7 @@ result.match({
 });
 
 // Clean up
-await client.close();
+await client.close().get();
 ```
 
 ## Error Handling