fix: fable_add_task_log sends content not body to match API

The MCP tool was sending {"body": ...} but the task logs API route
expects {"content": ...}, causing 400 errors.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

fable-mcp/fable_mcp/tools/tasks.py

@@ -87,7 +87,7 @@ async def add_task_log(
     client: FableClient,
     *,
     task_id: int,
-    body: str,
+    content: str,
 ) -> dict[str, Any]:
     """Append a log entry to a task."""
-    return await client.post(f"/api/tasks/{task_id}/logs", json={"body": body})
+    return await client.post(f"/api/tasks/{task_id}/logs", json={"content": content})