feat(coding-agent): auto-switch dark/light theme via SIGWINCH

Replace single `theme` setting with `theme.dark` and `theme.light`.
Auto-detection is always on — COLORFGBG determines which slot to use.
On SIGWINCH, re-check COLORFGBG and switch themes if background changed.
Old `theme` setting auto-migrated using luminance detection.

Fixes #65

packages/coding-agent/test/streaming-render-debug.ts

@@ -12,7 +12,7 @@ import { sleep } from "bun";
 
 // Initialize dark theme with full color support
 Bun.env.COLORTERM = "truecolor";
-initTheme("dark");
+initTheme();
 
 async function main() {
 	// Load the real fixture that caused the bug