feat: add transform node input key examples

Author: romashorodok

docs/user-guide/workflows/configuration/specialized-nodes.md

@@ -143,6 +143,71 @@ custom_nodes:
 | `on_error`       | string | No       | `fail`          | Error strategy: `fail`, `skip`, `default`, `partial`                            |
 | `default_output` | object | No       | `{}`            | Default output when `on_error: default`                                         |
 
+#### Input Key Path Extraction
+
+The `input_key` parameter controls how data is extracted from the context store and made available to your mappings.
+
+All examples below use this context store:
+
+```json
+{
+  "name": "Alice",
+  "age": 30,
+  "city": "NYC",
+  "user": {
+    "name": "Bob",
+    "age": 25
+  },
+  "api": {
+    "response": {
+      "data": {
+        "items": [1, 2, 3],
+        "total": 3
+      }
+    }
+  },
+  "stats": {
+    "total": {
+      "count": 42,
+      "sum": 1050,
+      "average": 25.0
+    }
+  },
+  "users": [
+    {"name": "Alice", "role": "admin"},
+    {"name": "Bob", "role": "user"}
+  ]
+}
+```
+
+##### Extraction Examples
+
+| Input Key | Extracted Value | Available Variables |
+|-----------|-----------------|---------------------|
+| *(null/omitted)* | Entire context | `name`=`"Alice"`, `age`=`30`, `city`=`"NYC"`, `user`=`{...}`, `api`=`{...}`, `stats`=`{...}`, `users`=`[...]` |
+| `age` | `30` | `age` = `30` |
+| `user` | `{"name": "Bob", "age": 25}` | `name` = `"Bob"`, `age` = `25` |
+| `users` | `[{"name": "Alice", ...}, ...]` | `users` = `[{"name": "Alice", "role": "admin"}, {"name": "Bob", "role": "user"}]` |
+| `api.response.data` | `{"items": [1,2,3], "total": 3}` | `data` = `{"items": [1, 2, 3], "total": 3}` |
+| `api.response.data.total` | `3` | `total` = `3` |
+| `stats.total` | `{"count": 42, "sum": 1050, ...}` | `total` = `{"count": 42, "sum": 1050, "average": 25.0}` |
+| `users[0]` | `{"name": "Alice", "role": "admin"}` | `users` = `{"name": "Alice", "role": "admin"}` |
+| `users[-1]` | `{"name": "Bob", "role": "user"}` | `users` = `{"name": "Bob", "role": "user"}` |
+| `users[0:2]` | `[{"name": "Alice", ...}, ...]` | `users` = `[{"name": "Alice", "role": "admin"}, {"name": "Bob", "role": "user"}]` |
+| `users[0].name` | `"Alice"` | `name` = `"Alice"` |
+| `users[1].role` | `"user"` | `role` = `"user"` |
+| `api.response.data.items[0]` | `1` | `items` = `1` |
+
+##### Behavior Rules
+
+1. **No key** (`null` or omitted): Entire context store merged - all top-level keys become variables
+2. **Simple key** (no dots):
+   - If value is **dict**: Contents unwrapped → dict keys become variables
+   - If value is **not dict**: Value wrapped with key name
+3. **Nested key** (with dots): Value always wrapped with **last part** of the path
+4. **Array indexing** (`[n]`, `[-n]`, `[start:end]`): Element/slice wrapped with **array name** (part before `[`)
+5. **Chained indexing** (e.g., `users[0].name`): Continues path after index, wraps with **last part** of remaining path
+
 #### Mapping Types
 
 Transform Node supports six mapping types:
@@ -219,6 +284,20 @@ Execute Python expressions with restricted namespace.
   script: "urgency * 2 + importance"
 ```
 
+**Available Python Builtins:**
+
+For security, scripts run in a restricted environment with only these Python builtins:
+
+| Category | Available |
+|----------|-----------|
+| **Constants** | `True`, `False`, `None` |
+| **Type Constructors** | `str`, `int`, `float`, `bool` |
+| **Common Functions** | `len`, `min`, `max`, `sum`, `abs` |
+| **Collections** | `list`, `dict`, `set`, `tuple` |
+| **Utilities** | `any`, `all`, `isinstance` |
+
+**Note:** Import statements, file I/O, and other Python builtins are not available for security reasons.
+
 #### Sequential Processing
 
 **Important:** Mappings are processed sequentially. Later mappings can reference fields created by earlier mappings.