Feature: expose envelope To / Delivered-To (or equivalent) in thread/message JSON

## Problem
When screening or auditing mail with the HEY CLI (`hey --json`), there is no reliable way to see the envelope recipient (SMTP `RCPT TO` / `Delivered-To` / alias that actually received the message). Billing portals and payment processors often show a different address than the mailbox you are looking at, and without envelope recipient data it is hard to confirm which alias or forward path a message hit.

## Request
Please expose in CLI JSON (thread and/or message payloads) at least one of:
- envelope `To` / `RCPT TO`
- `Delivered-To`
- the HEY alias / address that received the message

Even a read-only field would unblock scripting and support workflows.

## Context
Filed after hitting this while reconciling a billing receipt against mailbox state. Soft-delete / Trash via CLI works; hard-delete is out of scope here. This is only about recipient metadata visibility.

Start at the HEY CLI's `hey --json` entry point and trace how thread and message payloads are serialized. Inspect the existing recipient metadata, then expose an envelope recipient or equivalent received-address field and verify that it appears in the relevant JSON output for scripting and support workflows.

Written by the indexing model from the issue text.