Add setting to control diff context size in CLI edit previews

Problem

When Claude Code proposes a code edit in the CLI, the diff preview shows a small number of context lines around the change. For edits in the middle of a method or class, it can be hard to orient yourself without seeing more of the surrounding code.

Proposed solution

Add a setting (e.g. diffContextLines) to control how many lines of context are shown before and after each change hunk in the CLI edit preview. For example:

{
  "diffContextLines": 10
}

The default could remain as-is for users who prefer compact diffs, while users who want more context can increase it.

Alternatives considered

• Using an IDE extension where the native diff viewer shows full context - works but not always an option for CLI-first workflows
• Setting diffTool to an external viewer - adds friction to the approve/deny flow

Additional context

This would help with reviewing edits in larger files where the surrounding code is needed to understand whether the change is correct.