Add settings/arguments guidance to spec template

When proposing new settings, also propose corresponding CLI arguments
(and vice versa). Settings define user defaults; CLI arguments provide
per-invocation overrides.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: denelon

doc/specs/spec-template.md

@@ -20,6 +20,7 @@ issue id: <github issue id>
 ## Solution Design
 
 [comment]: # Outline the design of the solution. Feel free to include ASCII-art diagrams, etc.
+[comment]: # When proposing new settings, also propose corresponding CLI arguments (and vice versa). Settings define user defaults; CLI arguments provide per-invocation overrides.
 
 ## UI/UX Design