Initialize grasp_frame_transform to Identity in MTC tutorial

[remete618]

Summary

• Fixes MTC Pick and Place tutorial uses uninitialised value for grasp_frame_transform #1079
• Initializes grasp_frame_transform to Eigen::Isometry3d::Identity() in both the tutorial source code and the RST code block

What changed

Two files:

• src/mtc_node.cpp (line 191)
• pick_and_place_with_moveit_task_constructor.rst (line 699)

// Before (uninitialized, translation component contains garbage values)
Eigen::Isometry3d grasp_frame_transform;

// After
Eigen::Isometry3d grasp_frame_transform = Eigen::Isometry3d::Identity();

The original code set rotation via quaternion but never initialized the translation. Identity() zeroes the translation and sets rotation to identity before the quaternion assignment overwrites the rotation.

Test plan

• Tutorial code compiles
• RST code block matches source