feat: load channel plugins

docs/plugin.md

@@ -165,6 +165,45 @@ Plugins export either:
 - A function: `(api) => { ... }`
 - An object: `{ id, name, configSchema, register(api) { ... } }`
 
+### Register a messaging channel
+
+Plugins can register **channel plugins** that behave like built‑in channels
+(WhatsApp, Telegram, etc.). Channel config lives under `channels.<id>` and is
+validated by your channel plugin code.
+
+```ts
+const myChannel = {
+  id: "acmechat",
+  meta: {
+    id: "acmechat",
+    label: "AcmeChat",
+    selectionLabel: "AcmeChat (API)",
+    docsPath: "/channels/acmechat",
+    blurb: "demo channel plugin.",
+    aliases: ["acme"],
+  },
+  capabilities: { chatTypes: ["direct"] },
+  config: {
+    listAccountIds: (cfg) => Object.keys(cfg.channels?.acmechat?.accounts ?? {}),
+    resolveAccount: (cfg, accountId) =>
+      (cfg.channels?.acmechat?.accounts?.[accountId ?? "default"] ?? { accountId }),
+  },
+  outbound: {
+    deliveryMode: "direct",
+    sendText: async () => ({ ok: true }),
+  },
+};
+
+export default function (api) {
+  api.registerChannel({ plugin: myChannel });
+}
+```
+
+Notes:
+- Put config under `channels.<id>` (not `plugins.entries`).
+- `meta.label` is used for labels in CLI/UI lists.
+- `meta.aliases` adds alternate ids for normalization and CLI inputs.
+
 ### Register a tool
 
 ```ts