Add onTurn() to API quick reference, fix jsonc language tag in SKILL.md

Add missing onTurn() convenience method to the API Quick Reference table
and change the fenced code block language tag from json to jsonc for the
multi-turn fixture examples that contain // comments.

Author: jpr5

skills/write-fixtures/SKILL.md

@@ -53,7 +53,7 @@ Multi-part content (e.g., `[{type: "text", text: "hello"}]`) is automatically ex
 
 ### Multi-turn fixture examples
 
-```json
+```jsonc
 // 2-step HITL with turnIndex
 {"match": {"userMessage": "trip to mars", "turnIndex": 0}, "response": {"toolCalls": [{"id": "call_001", "name": "generate_steps", "arguments": "{}"}]}}
 {"match": {"userMessage": "trip to mars", "turnIndex": 1}, "response": {"content": "Great choices! Proceeding."}}
@@ -691,6 +691,7 @@ const mock = await LLMock.create({ port: 0 }); // creates + starts in one call
 | `onJsonOutput(pattern, json, opts?)`    | Match by user message with `responseFormat` |
 | `onToolCall(name, response, opts?)`     | Match by tool name in `tools[]`             |
 | `onToolResult(id, response, opts?)`     | Match by `tool_call_id`                     |
+| `onTurn(turn, pattern, response, opts?)` | Match by turn index + user message          |
 | `nextRequestError(status, body?)`       | One-shot error, auto-removes                |
 | `loadFixtureFile(path)`                 | Load JSON fixture file                      |
 | `loadFixtureDir(path)`                  | Load all JSON files in directory            |