Feature Request: Show plan mode changes as diff view

Summary

When reviewing plan updates in plan mode, it would be helpful to see changes as a diff rather than having to re-read the entire plan from the top.

Problem

Currently, when a plan is modified in plan mode, the user sees the full updated plan. For longer plans, this requires re-reading the entire document to identify what changed, which is time-consuming and error-prone.

Proposed Solution

Display plan mode changes in a diff view format, highlighting:

• Added sections/steps (green)
• Removed sections/steps (red)
• Modified content (yellow/changed)

This would allow users to quickly understand exactly what was updated without scanning the entire plan.

Benefits

• Faster review cycles for plan iterations
• Clearer understanding of what changed between versions
• Reduced cognitive load when reviewing complex plans
• Consistent with how code changes are already shown in diffs