[FEATURE]: Replace tui.diff_style with KV backed storage

[ariane-emory]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

When I originally added the tui.diff_style setting a few months back, I was not yet familiar enough with the project to have been aware of the kv.json file. I have by now realized that KV storage would indeed be a more apt way to store the user's preferred diffStyle, since it allows for easier dynamic toggling and a cleaner configuration file.

Verdict: the diff_style property ought be moved to kv.json, and menu items in the command_list provided to toggle its value while Opencode is live.

[github-actions]

This feature request aligns with several related discussions in the repository about dynamic configuration and TUI settings management. Please consider checking these related issues:

• [FEATURE]: Add tui.tips configuration option to disable input placeholder hints #10571: Add tui.tips configuration option to disable input placeholder hints
• [FEATURE]: Add hints toggle to declutter UI #6240: Add hints toggle to declutter UI (proposes KV-backed toggling)
• [FEATURE]: add "/experimental" command to toggle experimental features #7717: Add "/experimental" command to toggle experimental features (similar toggling pattern)
• [FEATURE]: Light/Dark mode auto-switching #9697: Light/Dark mode auto-switching (similar dynamic switching pattern)

These issues share common themes around:

1. Moving TUI settings to more flexible storage (currently in opencode.json)
2. Providing runtime/dynamic toggling of features via commands
3. Improving configuration flexibility

Feel free to check these to ensure the implementation approach aligns with the broader architecture for configuration management in OpenCode.