[FEATURE] Plan mode: cycle between full plan, diff only, and highlighted view with arrow keys

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When I ask for a change to a plan, Claude shows the whole plan again. I have to re-read all of it to find what changed.

Proposed Solution

In plan mode, the left and right arrow keys cycle through three views:

1. Full: the whole plan, as it is today.
2. Diff only: just what changed since the previous turn.
3. Both: the whole plan, with unchanged parts greyed out and changed parts in the normal text color. Should be default IMHO.

Why: On long plans I only need to judge what changed. Greyed context keeps my place in the plan without making me re-read it.

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

Related, closed without a fix: #16933, #36075, #58213.