fix: harden Mattermost plugin gating (#1428) (thanks @damoahdominic)

docs/channels/mattermost.md

@@ -5,12 +5,33 @@ read_when:
   - Debugging Mattermost routing
 ---
 
-# Mattermost
+# Mattermost (plugin)
+
+Status: supported via plugin (bot token + WebSocket events). Channels, groups, and DMs are supported.
+
+## Plugin required
+Mattermost ships as a plugin and is not bundled with the core install.
+
+Install via CLI (npm registry):
+```bash
+clawdbot plugins install @clawdbot/mattermost
+```
+
+Local checkout (when running from a git repo):
+```bash
+clawdbot plugins install ./extensions/mattermost
+```
+
+If you choose Mattermost during configure/onboarding and a git checkout is detected,
+Clawdbot will offer the local install path automatically.
+
+Details: [Plugins](/plugin)
 
 ## Quick setup
-1) Create a Mattermost bot account and copy the **bot token**.
-2) Copy the Mattermost **base URL** (e.g., `https://chat.example.com`).
-3) Configure Clawdbot and start the gateway.
+1) Install the Mattermost plugin.
+2) Create a Mattermost bot account and copy the **bot token**.
+3) Copy the Mattermost **base URL** (e.g., `https://chat.example.com`).
+4) Configure Clawdbot and start the gateway.
 
 Minimal config:
 ```json5
@@ -19,7 +40,8 @@ Minimal config:
     mattermost: {
       enabled: true,
       botToken: "mm-token",
-      baseUrl: "https://chat.example.com"
+      baseUrl: "https://chat.example.com",
+      dmPolicy: "pairing"
     }
   }
 }
@@ -56,6 +78,18 @@ Notes:
 - `onchar` still responds to explicit @mentions.
 - `channels.mattermost.requireMention` is honored for legacy configs but `chatmode` is preferred.
 
+## Access control (DMs)
+- Default: `channels.mattermost.dmPolicy = "pairing"` (unknown senders get a pairing code).
+- Approve via:
+  - `clawdbot pairing list mattermost`
+  - `clawdbot pairing approve mattermost <CODE>`
+- Public DMs: `channels.mattermost.dmPolicy="open"` plus `channels.mattermost.allowFrom=["*"]`.
+
+## Channels (groups)
+- Default: `channels.mattermost.groupPolicy = "allowlist"` (mention-gated).
+- Allowlist senders with `channels.mattermost.groupAllowFrom` (user IDs or `@username`).
+- Open channels: `channels.mattermost.groupPolicy="open"` (mention-gated).
+
 ## Targets for outbound delivery
 Use these target formats with `clawdbot message send` or cron/webhooks: