diff: add configurable half-page scrolling keybindings

Summary

The diff viewer has no configurable actions for scrolling up or down by half a page. This prevents Vim-style ctrl+u and ctrl+d navigation even though session navigation already provides equivalent half-page actions.

Environment

• opencode version: v0.0.0-beta-17519
• OS: Linux 7.1.8-arch1-3 x86_64 (Arch Linux)
• Terminal: TERM=xterm-256color; TERM_PROGRAM and COLORTERM unavailable
• Shell: /usr/bin/fish
• Install/channel: beta
• Active plugins: local plugin /home/pedro/.config/opencode/plugins/notifier.js

Reproduction

1. Open the diff viewer.
2. Try to configure Vim-style half-page scrolling using ctrl+d and ctrl+u.
3. Check the available diff-viewer command IDs in the keybinding configuration.

Expected Behavior

The diff viewer should expose configurable half-page actions, such as diff.half.page.down and diff.half.page.up, allowing bindings like ctrl+d and ctrl+u.

Actual Behavior

The diff viewer exposes line scrolling (diff.down and diff.up) and full-page scrolling (diff.page.down and diff.page.up), but no half-page scrolling actions. The session view has session.half.page.down and session.half.page.up, so the behavior is inconsistent between views.

Additional Context

The documented diff keybindings currently use ctrl+f and ctrl+b for full-page scrolling. This request is specifically for configurable half-page navigation in the diff viewer, not general keybinding configuration.