fix(renderer): respect alpha channel for transparent background

[Stranmor]

Summary

• Fix transparent background not working when theme specifies "background": "none"
• Check alpha channel before emitting ANSI RGB background escape codes

Problem

When backgroundColor has alpha = 0 (transparent), the renderer was still calling ANSI.setRgbBackground(), which overrides the terminal's native transparency settings with a solid color (typically black).

Solution

Added a simple alpha check before setting RGB background:

if (backgroundColor[3] === 0) {
  clear = newlines
} else {
  clear = ANSI.setRgbBackground(...) + newlines + ANSI.resetBackground
}

Test plan

• Create custom theme with "background": { "dark": "none", "light": "none" }
• Apply theme in opencode
• Verify terminal's transparent background shows through

Tested with:

• Terminal: Kitty (GPU-accelerated with transparency support)
• OS: Arch Linux

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