feat: add NO_COLOR environment variable support (#122)

Add a centralized color module (cli/src/color.rs) that respects the
NO_COLOR environment variable per https://no-color.org/

Changes:
- Add color.rs module with helper functions for colored output
- Refactor all hardcoded ANSI escape codes to use the color module
- Add tests for color formatting functions
- Update AGENTS.md with color module usage guidelines

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

AGENTS.md

@@ -5,6 +5,7 @@ Instructions for AI coding agents working with this codebase.
 ## Code Style
 
 - Do not use emojis in code, output, or documentation. Unicode symbols (✓, ✗, →, ⚠) are acceptable.
+- CLI colored output uses `cli/src/color.rs`. This module respects the `NO_COLOR` environment variable. Never use hardcoded ANSI color codes.
 
 <!-- opensrc:start -->