docs(channels): add plugin developer guide and rename mock to plugin-example

- Add comprehensive developer guide for building channel plugins
- Add user-facing docs for installing/configuring custom channel plugins
- Replace custom-channels.md with new plugins.md
- Rename @qwen-code/channel-mock to @qwen-code/channel-plugin-example
- Add messageId field to Envelope type for response correlation

This provides clear documentation for developers building custom channel
adapters and renames the mock package to better reflect its purpose as
a reference implementation example.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

docs/users/features/channels/plugins.md

@@ -0,0 +1,87 @@
+# Custom Channel Plugins
+
+You can extend the channel system with custom platform adapters packaged as [extensions](../../extension/introduction). This lets you connect Qwen Code to any messaging platform, webhook, or custom transport.
+
+## How It Works
+
+Channel plugins are loaded at startup from active extensions. When `qwen channel start` runs, it:
+
+1. Scans all enabled extensions for `channels` entries in their `qwen-extension.json`
+2. Dynamically imports each channel's entry point
+3. Registers the channel type so it can be referenced in `settings.json`
+4. Creates channel instances using the plugin's factory function
+
+Your custom channel gets the full shared pipeline for free: sender gating, group policies, session routing, slash commands, crash recovery, and the ACP bridge to the agent.
+
+## Installing a Custom Channel
+
+Install an extension that provides a channel plugin:
+
+```bash
+# From a local path (for development or private plugins)
+qwen extensions install /path/to/my-channel-extension
+
+# Or link it for development (changes are reflected immediately)
+qwen extensions link /path/to/my-channel-extension
+```
+
+## Configuring a Custom Channel
+
+Add a channel entry to `~/.qwen/settings.json` using the custom type provided by the extension:
+
+```json
+{
+  "channels": {
+    "my-bot": {
+      "type": "my-platform",
+      "apiKey": "$MY_PLATFORM_API_KEY",
+      "senderPolicy": "open",
+      "cwd": "/path/to/project"
+    }
+  }
+}
+```
+
+The `type` must match a channel type registered by an installed extension. Check the extension's documentation for which plugin-specific fields are required (e.g., `apiKey`, `webhookUrl`).
+
+All standard channel options work with custom channels:
+
+| Option         | Description                                    |
+| -------------- | ---------------------------------------------- |
+| `senderPolicy` | `allowlist`, `pairing`, or `open`              |
+| `allowedUsers` | Static allowlist of sender IDs                 |
+| `sessionScope` | `user`, `thread`, or `single`                  |
+| `cwd`          | Working directory for the agent                |
+| `instructions` | Prepended to the first message of each session |
+| `model`        | Model override for the channel                 |
+| `groupPolicy`  | `disabled`, `allowlist`, or `open`             |
+| `groups`       | Per-group settings                             |
+
+See [Overview](./overview) for details on each option.
+
+## Starting the Channel
+
+```bash
+# Start all channels including custom ones
+qwen channel start
+
+# Start just your custom channel
+qwen channel start my-bot
+```
+
+## What You Get for Free
+
+Custom channels automatically support everything built-in channels do:
+
+- **Sender policies** — `allowlist`, `pairing`, and `open` access control
+- **Group policies** — Per-group settings with optional @mention gating
+- **Session routing** — Per-user, per-thread, or single shared sessions
+- **DM pairing** — Full pairing code flow for unknown users
+- **Slash commands** — `/help`, `/clear`, `/status` work out of the box
+- **Custom instructions** — Prepended to the first message in each session
+- **Crash recovery** — Automatic restart with session preservation
+- **Per-session serialization** — Messages are queued to prevent race conditions
+
+## Building Your Own Channel Plugin
+
+Want to build a channel plugin for a new platform? See the [Channel Plugin Developer Guide](/developers/channel-plugins) for the `ChannelPlugin` interface, the `Envelope` format, and extension points.