feat: add per-group allowedUsers filtering for all channels (#283)

docs/configuration.md

@@ -271,6 +271,7 @@ Use `groups.<id>.mode` to control how each group/channel behaves:
 - `open`: process and respond to all messages (default behavior)
 - `listen`: process all messages for context/memory, only respond when mentioned
 - `mention-only`: drop group messages unless the bot is mentioned
+- `disabled`: drop all group messages unconditionally, even if the bot is mentioned
 
 You can also use `*` as a wildcard default:
 
@@ -283,6 +284,39 @@ channels:
       "-1009876543210": { mode: mention-only }
 ```
 
+### Per-Group User Filtering
+
+Use `groups.<id>.allowedUsers` to restrict which users can trigger the bot in a specific group. When set, messages from users not in the list are silently dropped before reaching the agent (no token cost).
+
+```yaml
+channels:
+  discord:
+    groups:
+      "*":
+        mode: mention-only
+        allowedUsers:
+          - "123456789012345678"     # Only this user triggers the bot
+      "TESTING_CHANNEL":
+        mode: open
+        # No allowedUsers -- anyone can interact in this channel
+```
+
+Resolution follows the same priority as `mode`: specific channel/group ID > guild/server ID > `*` wildcard. Omitting `allowedUsers` means all users are allowed.
+
+This works across all channels (Discord, Telegram, Slack, Signal, WhatsApp).
+
+### Finding Group IDs
+
+Each channel uses different identifiers for groups:
+
+- **Telegram**: Group IDs are negative numbers (e.g., `-1001234567890`). To find one: add `@userinfobot` to the group, or forward a group message to `@userinfobot`. You can also check the bot logs -- group IDs are printed when the bot receives a message.
+- **Discord**: Channel and server IDs are numeric strings (e.g., `123456789012345678`). Enable **Developer Mode** in Discord settings (User Settings > Advanced > Developer Mode), then right-click any channel or server and select "Copy Channel ID" or "Copy Server ID".
+- **Slack**: Channel IDs start with `C` (e.g., `C01ABC23DEF`). Right-click a channel > "View channel details" > scroll to the bottom to find the Channel ID.
+- **WhatsApp**: Group JIDs look like `120363123456@g.us`. These appear in the bot logs when the bot receives a group message.
+- **Signal**: Group IDs appear in the bot logs on first group message. Use the `group:` prefix in config (e.g., `group:abc123`).
+
+**Tip**: If you don't know the ID yet, start the bot with `"*": { mode: mention-only }`, send a message in the group, and check the logs for the ID.
+
 Deprecated formats are still supported and auto-normalized with warnings:
 
 - `listeningGroups: ["id"]` -> `groups: { "id": { mode: listen } }`