feat(skill): add local n8n webhook skill draft

skills/n8n-webhook/references/payloads.md

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+# n8n-webhook payload notes
+
+## Current live endpoint
+
+### `openclaw-ping`
+
+Purpose:
+- confirm OpenClaw can reach the local n8n webhook surface end-to-end
+
+Typical request body:
+
+```json
+{
+  "message": "hello from OpenClaw"
+}
+```
+
+Recommended success response:
+
+```json
+{
+  "ok": true,
+  "service": "n8n-agent",
+  "message": "openclaw webhook reached"
+}
+```
+
+## Recommended contract for new endpoints
+
+Prefer a stable JSON shape like:
+
+```json
+{
+  "ok": true,
+  "request_id": "optional-uuid",
+  "result": {
+    "...": "workflow-specific payload"
+  }
+}
+```
+
+On failure, return a non-2xx status plus:
+
+```json
+{
+  "ok": false,
+  "error": {
+    "code": "short-machine-code",
+    "message": "human-readable summary"
+  }
+}
+```
+
+## Naming guidance
+
+- Use lowercase kebab-case webhook paths.
+- Keep names explicit: `openclaw-ping`, `openclaw-action`, `append-log`, `send-notify`.
+- Avoid generic names like `run`, `task`, or `webhook1`.
+
+## Suggested action-bus pattern
+
+If multiple agent-safe actions are needed, prefer one router webhook such as `openclaw-action`.
+
+Request shape:
+
+```json
+{
+  "action": "append_log",
+  "args": {
+    "text": "backup finished"
+  },
+  "request_id": "optional-uuid"
+}
+```
+
+That keeps the external surface small while letting n8n route internally.