dsh-mail-channel: Mail channel for DeepSeek Harness

dsh-mail-channel provides IMAP/SMTP mail tools, sidebar toggles, and arrival wake-up for DeepSeek Harness; it is not an email client. It supports one-line `mail_setup` configuration for conversations, read-to-claim, and `mail_verify` verification. This article introduces installation commands, a three-step quick start, reply channel rules, and optional configurations such as `sendPolicy` based on the official README, and explains notes regarding single mailbox usage and process permissions.