Feature: external diff viewer alongside CLI approvals

Feature request

Add a diffTool config that opens diffs in an external viewer (e.g. IntelliJ, opendiff) in addition to showing them in the CLI.

The diff should appear in both places simultaneously - CLI for approving/rejecting, external viewer for a richer visual read.

Why this matters:

• For simple edits, users can approve straight from the CLI without switching screens.
• For complex edits, the external viewer gives a better place to read the full diff without losing the CLI approval flow.

Current problem:

The JetBrains plugin takes over entirely - it moves both the diff display AND the approval flow into the IDE, forcing users to switch contexts for every edit regardless of complexity. There is no way to use the IDE as a read-only diff viewer while keeping the CLI as the primary approval interface.

Proposed behavior:

A config option (e.g. diffTool: "idea diff") that launches an external diff viewer when Claude proposes an edit, while still showing the diff inline in the CLI and keeping the approve/reject flow in the terminal.