fix(mattermost): harden DM channel retry handling

docs/channels/mattermost.md

@@ -191,6 +191,35 @@ OpenClaw resolves them **user-first**:
 
 If you need deterministic behavior, always use the explicit prefixes (`user:<id>` / `channel:<id>`).
 
+## DM channel retry
+
+When OpenClaw sends to a Mattermost DM target and needs to resolve the direct channel first, it
+retries transient direct-channel creation failures by default.
+
+Use `channels.mattermost.dmChannelRetry` to tune that behavior globally for the Mattermost plugin,
+or `channels.mattermost.accounts.<id>.dmChannelRetry` for one account.
+
+```json5
+{
+  channels: {
+    mattermost: {
+      dmChannelRetry: {
+        maxRetries: 3,
+        initialDelayMs: 1000,
+        maxDelayMs: 10000,
+        timeoutMs: 30000,
+      },
+    },
+  },
+}
+```
+
+Notes:
+
+- This applies only to DM channel creation (`/api/v4/channels/direct`), not every Mattermost API call.
+- Retries apply to transient failures such as rate limits, 5xx responses, and network or timeout errors.
+- 4xx client errors other than `429` are treated as permanent and are not retried.
+
 ## Reactions (message tool)
 
 - Use `message action=react` with `channel=mattermost`.