docs: add messaging channel guide

docs/plugin.md

@@ -204,6 +204,82 @@ Notes:
 - `meta.label` is used for labels in CLI/UI lists.
 - `meta.aliases` adds alternate ids for normalization and CLI inputs.
 
+### Write a new messaging channel (step‑by‑step)
+
+Use this when you want a **new chat surface** (a “messaging channel”), not a model provider.
+Model provider docs live under `/providers/*`.
+
+1) Pick an id + config shape
+- All channel config lives under `channels.<id>`.
+- Prefer `channels.<id>.accounts.<accountId>` for multi‑account setups.
+
+2) Define the channel metadata
+- `meta.label`, `meta.selectionLabel`, `meta.docsPath`, `meta.blurb` control CLI/UI lists.
+- `meta.docsPath` should point at a docs page like `/channels/<id>`.
+
+3) Implement the required adapters
+- `config.listAccountIds` + `config.resolveAccount`
+- `capabilities` (chat types, media, threads, etc.)
+- `outbound.deliveryMode` + `outbound.sendText` (for basic send)
+
+4) Add optional adapters as needed
+- `setup` (wizard), `security` (DM policy), `status` (health/diagnostics)
+- `gateway` (start/stop/login), `mentions`, `threading`, `streaming`
+- `actions` (message actions), `commands` (native command behavior)
+
+5) Register the channel in your plugin
+- `api.registerChannel({ plugin })`
+
+Minimal config example:
+
+```json5
+{
+  channels: {
+    acmechat: {
+      accounts: {
+        default: { token: "ACME_TOKEN", enabled: true }
+      }
+    }
+  }
+}
+```
+
+Minimal channel plugin (outbound‑only):
+
+```ts
+const plugin = {
+  id: "acmechat",
+  meta: {
+    id: "acmechat",
+    label: "AcmeChat",
+    selectionLabel: "AcmeChat (API)",
+    docsPath: "/channels/acmechat",
+    blurb: "AcmeChat messaging channel.",
+    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 ({ text }) => {
+      // deliver `text` to your channel here
+      return { ok: true };
+    },
+  },
+};
+
+export default function (api) {
+  api.registerChannel({ plugin });
+}
+```
+
+Load the plugin (extensions dir or `plugins.load.paths`), restart the gateway,
+then configure `channels.<id>` in your config.
+
 ### Register a tool
 
 ```ts