feat(guide): add Invocation of External Scripts section

lib/guide/lang-reuse.md

@@ -131,3 +131,79 @@ system: |-
 
   {{description}}
 ```
+
+## Invocation of External Scripts
+
+### Chains Invocation of Agent Scripts Or Instructions
+
+Within messages, results can be forwarded to other agents.
+
+If no parameters are specified, the AI outcome will be passed as the `content` parameter to the agent. For instance,
+
+`list-expression.ai.yaml`:
+
+```yaml
+system: Only list the calculation expression, do not calculate the result
+---
+user: "Three candies plus five candies."
+assistant: "[[CalcExpression]]"
+# The actual input to the agent in this case is: {content: "[AI-generated calculation expression]"}
+-> calculator
+$echo: "#A total of {{LatestResult}} pieces of candy"
+```
+
+`calculator.ai.yaml`:
+
+
+```yaml
+---
+# Below is the front-matter configuration
+parameters:
+  response_format:
+    type: "json"
+output:
+  type: "number"
+---
+# Below is the script
+system: Please as a calculator to calculate the result of the following expression. Only output the result.
+--- # mark the beginning of new dialogue
+user: "{{content}}"
+```
+
+When parameters are included, the AI `content` is combined with these parameters and forwarded together to the agent. For example,
+
+```yaml
+user: "Tell me a joke!"
+assistant: "[[JOKE]]"
+# The actual input to the agent here is: {content: "[This is a joke generated by AI]", target_lang: "Portuguese"}
+-> translator(target_lang="Portuguese") -> $print
+```
+
+**Note**:
+
+* Call internal instruction with `$` prefix
+* If the script returns a value of type `string`/`boolean`/`number`, that return value will be placed to the `content` field. If the return value is an `object`, its contents will be directly passed to the agent.
+
+### External Script Invocation Formatting
+
+In messages, we support content substitution by invoking scripts or instructions. The script or instructions must return a string value. For example:
+
+```yaml
+# the `#` prefix means immediate formatting
+# call the `calculator.ai.yaml` script with the parameter '5+2'
+user: "#five plus two equals @calculator('5+2')"
+```
+
+Notes:
+
+* The prefix `#` indicates immediate formatting of the string.
+* The prefix `@` indicates calling an external script with the ID `calculator`. if there are no parameters, you must omit the parentheses.
+* If placed within text, ensure there is at least one space before and after. Extra spaces will be removed after substitution.
+
+Here’s an example of how to load a file and generate a summary using this method:
+
+```yaml
+user: |-
+  Generate a summary for the following file:
+  @file(file.txt)
+```