docs: update AGENTS.md, README.md, and messaging.md to include interactive setup for messaging platforms

Enhanced documentation to reflect the new interactive setup command for configuring messaging platforms (Telegram, Discord, Slack, WhatsApp). Updated sections in AGENTS.md, README.md, and messaging.md to provide clear instructions on using the 'hermes gateway setup' command, improving user experience and accessibility for platform configuration.

AGENTS.md

@@ -235,6 +235,7 @@ The unified `hermes` command provides all functionality:
 | `hermes update` | Update to latest (checks for new config) |
 | `hermes uninstall` | Uninstall (can keep configs for reinstall) |
 | `hermes gateway` | Start gateway (messaging + cron scheduler) |
+| `hermes gateway setup` | Configure messaging platforms interactively |
 | `hermes gateway install` | Install gateway as system service |
 | `hermes cron list` | View scheduled jobs |
 | `hermes cron status` | Check if cron scheduler is running |
@@ -245,7 +246,19 @@ The unified `hermes` command provides all functionality:
 
 ## Messaging Gateway
 
-The gateway connects Hermes to Telegram, Discord, and WhatsApp.
+The gateway connects Hermes to Telegram, Discord, Slack, and WhatsApp.
+
+### Setup
+
+The interactive setup wizard handles platform configuration:
+
+```bash
+hermes gateway setup      # Arrow-key menu of all platforms, configure tokens/allowlists/home channels
+```
+
+This is the recommended way to configure messaging. It shows which platforms are already set up, walks through each one interactively, and offers to start/restart the gateway service at the end.
+
+Platforms can also be configured manually in `~/.hermes/.env`:
 
 ### Configuration (in `~/.hermes/.env`):