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>

cli/src/color.rs

@@ -0,0 +1,158 @@
+//! Color output utilities respecting NO_COLOR environment variable.
+//!
+//! When the NO_COLOR environment variable is present (regardless of value),
+//! all color formatting is disabled per https://no-color.org/
+
+use std::env;
+use std::sync::OnceLock;
+
+/// Returns true if color output is enabled (NO_COLOR is NOT set)
+pub fn is_enabled() -> bool {
+    static COLORS_ENABLED: OnceLock<bool> = OnceLock::new();
+    *COLORS_ENABLED.get_or_init(|| env::var("NO_COLOR").is_err())
+}
+
+/// Format text in red (errors)
+pub fn red(text: &str) -> String {
+    if is_enabled() {
+        format!("\x1b[31m{}\x1b[0m", text)
+    } else {
+        text.to_string()
+    }
+}
+
+/// Format text in green (success)
+pub fn green(text: &str) -> String {
+    if is_enabled() {
+        format!("\x1b[32m{}\x1b[0m", text)
+    } else {
+        text.to_string()
+    }
+}
+
+/// Format text in yellow (warnings)
+pub fn yellow(text: &str) -> String {
+    if is_enabled() {
+        format!("\x1b[33m{}\x1b[0m", text)
+    } else {
+        text.to_string()
+    }
+}
+
+/// Format text in cyan (info/progress)
+pub fn cyan(text: &str) -> String {
+    if is_enabled() {
+        format!("\x1b[36m{}\x1b[0m", text)
+    } else {
+        text.to_string()
+    }
+}
+
+/// Format text in bold
+pub fn bold(text: &str) -> String {
+    if is_enabled() {
+        format!("\x1b[1m{}\x1b[0m", text)
+    } else {
+        text.to_string()
+    }
+}
+
+/// Format text in dim
+pub fn dim(text: &str) -> String {
+    if is_enabled() {
+        format!("\x1b[2m{}\x1b[0m", text)
+    } else {
+        text.to_string()
+    }
+}
+
+/// Red X error indicator
+pub fn error_indicator() -> &'static str {
+    static INDICATOR: OnceLock<String> = OnceLock::new();
+    INDICATOR.get_or_init(|| {
+        if is_enabled() {
+            "\x1b[31m✗\x1b[0m".to_string()
+        } else {
+            "✗".to_string()
+        }
+    })
+}
+
+/// Green checkmark success indicator
+pub fn success_indicator() -> &'static str {
+    static INDICATOR: OnceLock<String> = OnceLock::new();
+    INDICATOR.get_or_init(|| {
+        if is_enabled() {
+            "\x1b[32m✓\x1b[0m".to_string()
+        } else {
+            "✓".to_string()
+        }
+    })
+}
+
+/// Yellow warning indicator
+pub fn warning_indicator() -> &'static str {
+    static INDICATOR: OnceLock<String> = OnceLock::new();
+    INDICATOR.get_or_init(|| {
+        if is_enabled() {
+            "\x1b[33m⚠\x1b[0m".to_string()
+        } else {
+            "⚠".to_string()
+        }
+    })
+}
+
+/// Get console log color prefix by level
+pub fn console_level_prefix(level: &str) -> String {
+    if !is_enabled() {
+        return format!("[{}]", level);
+    }
+
+    let color = match level {
+        "error" => "\x1b[31m",
+        "warning" => "\x1b[33m",
+        "info" => "\x1b[36m",
+        _ => "",
+    };
+    if color.is_empty() {
+        format!("[{}]", level)
+    } else {
+        format!("{}[{}]\x1b[0m", color, level)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_red_contains_ansi_codes() {
+        // Test the format structure (actual color depends on NO_COLOR env)
+        let formatted = format!("\x1b[31m{}\x1b[0m", "error");
+        assert!(formatted.contains("\x1b[31m"));
+        assert!(formatted.contains("\x1b[0m"));
+    }
+
+    #[test]
+    fn test_green_contains_ansi_codes() {
+        let formatted = format!("\x1b[32m{}\x1b[0m", "success");
+        assert!(formatted.contains("\x1b[32m"));
+    }
+
+    #[test]
+    fn test_console_level_prefix_contains_level() {
+        // Regardless of color state, the level text should be present
+        assert!(console_level_prefix("error").contains("error"));
+        assert!(console_level_prefix("warning").contains("warning"));
+        assert!(console_level_prefix("info").contains("info"));
+        assert!(console_level_prefix("log").contains("log"));
+    }
+
+    #[test]
+    fn test_indicators_contain_symbols() {
+        // Regardless of color state, symbols should be present
+        assert!(error_indicator().contains('✗'));
+        assert!(success_indicator().contains('✓'));
+        assert!(warning_indicator().contains('⚠'));
+    }
+}