docs(design): align auto-detection section with actual initColorState implementation

The design doc claimed explicit os.Stdout.IsTerminal() / os.Stderr.IsTerminal()
checks, but the implementation only checks NO_COLOR and delegates TTY detection
to fatih/color defaults. Updated the doc to reflect the actual approach.

Signed-off-by: spencercjh <spencercjh@gmail.com>

Author: spencercjh

docs/plans/2026-03-30-cli-ux-improvements-design.md

@@ -74,9 +74,9 @@ Register `ValidArgsFunction` on enum-type flags for shell completion:
   - `Hintf(format, args...)` — prints cyan hint prefix + yellow content
   - `Skipf(format, args...)` — prints dim skip message
 - Auto-detection:
-  - Respect `NO_COLOR` environment variable (https://no-color.org/)
-  - Check `os.Stdout.IsTerminal()` / `os.Stderr.IsTerminal()` — disable color when piped
-  - Use `color.NoColor` from fatih/color which handles both
+  - Respect `NO_COLOR` environment variable (https://no-color.org/) — mere presence disables color
+  - Otherwise rely on `fatih/color` defaults for terminal detection and color enable/disable behavior
+  - Initialize `color.NoColor` in `internal/cli/initColorState()` instead of performing explicit `IsTerminal` checks
 
 ### Output Architecture