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/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Changed
+
+- Light theme colors adjusted for WCAG AA compliance (4.5:1 contrast ratio against white backgrounds)
+
 ### Added
 
 - Extension example: `summarize.ts` for summarizing conversations using custom UI and an external model