fix(cli): drop unused supports_color import in _imecore_prompt

Ruff F401 surfaced this on the diagnostic CI matrix; the symbol is
imported from _branding but never referenced in the module.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: n-papaioannou

ifixai/cli/_imecore_prompt.py

@@ -5,7 +5,7 @@
 
 import click
 
-from ifixai.cli._branding import _ACCENT_RGB, _DIM_RGB, _truecolor, supports_color
+from ifixai.cli._branding import _ACCENT_RGB, _DIM_RGB, _truecolor
 
 ENV_NO_PROMPT = "IFIXAI_NO_PROMPT"