docs: document Files view icon setting

[vycdev2]

Summary

• document the todo.file.view.icons setting in the README settings example

Verification

• npm run format:check — passed: all matched files use Prettier formatting
• npm run compile — passed: webpack development build completed
• npm test — passed: 229 tests
• git diff --check upstream/develop...HEAD — passed

Risk

• Low: documentation-only change describing an existing setting and its current default.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.