docs: add theme hot-reloading section with SIGUSR2 signal

[skylight74]

Summary

Adds documentation for the undocumented theme hot-reloading feature using the SIGUSR2 signal.

This is useful for users who:

• Use dynamic theming tools like wallust or pywal
• Want to integrate OpenCode with wallpaper-based color schemes
• Need to test custom theme changes in real-time without restarting

Changes

• Added new "Theme hot-reloading" section to themes.mdx
• Documented the SIGUSR2 signal behavior
• Added example integration with wallust

How I verified it works

Discovered this feature by reading the source code in packages/opencode/src/cli/cmd/tui/context/theme.tsx which contains:

process.on("SIGUSR2", async () => {
  renderer.clearPaletteCache()
  init()
})

Tested by sending pkill -USR2 -x opencode after modifying theme files - themes reload correctly.