Improve light theme color contrast for WCAG compliance (#682)

Adjust base colors (teal, blue, green, red, yellow, dimGray) to meet
4.5:1 contrast ratio against white backgrounds. Update thinking level
colors to reference theme vars for consistency.

Refactor test-theme-colors.ts into a CLI with contrast, test, and theme
commands for easier color validation.

Co-authored-by: Mario Zechner <badlogicgames@gmail.com>

packages/coding-agent/src/modes/interactive/theme/light.json

@@ -2,13 +2,13 @@
 	"$schema": "https://raw.githubusercontent.com/badlogic/pi-mono/main/packages/coding-agent/theme-schema.json",
 	"name": "light",
 	"vars": {
-		"teal": "#5f8787",
-		"blue": "#5f87af",
-		"green": "#87af87",
-		"red": "#af5f5f",
-		"yellow": "#d7af5f",
+		"teal": "#5a8080",
+		"blue": "#547da7",
+		"green": "#588458",
+		"red": "#aa5555",
+		"yellow": "#9a7326",
 		"mediumGray": "#6c6c6c",
-		"dimGray": "#8a8a8a",
+		"dimGray": "#767676",
 		"lightGray": "#b0b0b0",
 		"selectedBg": "#d0d0e0",
 		"userMsgBg": "#e8e8e8",
@@ -68,9 +68,9 @@
 		"syntaxPunctuation": "#000000",
 
 		"thinkingOff": "lightGray",
-		"thinkingMinimal": "#9e9e9e",
-		"thinkingLow": "#5f87af",
-		"thinkingMedium": "#5f8787",
+		"thinkingMinimal": "#767676",
+		"thinkingLow": "blue",
+		"thinkingMedium": "teal",
 		"thinkingHigh": "#875f87",
 		"thinkingXhigh": "#8b008b",