Add reusable segmented inbox filters

AGENTS.md

@@ -34,6 +34,22 @@ or revise a reusable design decision do not require a documentation update.
 
 If uncertain whether a visual change is significant, update the design system.
 
+## Reusable interface patterns
+
+Favor shared, generic components whenever a visual or interaction pattern is
+used in more than one place or is likely to recur. Before adding page-local
+markup and styles, check `src/lib/components` and extend an appropriate shared
+component when practical. When introducing a reusable pattern, migrate the
+clearest existing duplicate in the same change so the component has more than
+one real consumer.
+
+For small, finite option sets, prefer the shared segmented-choice pattern over
+a dropdown. Use radio semantics for exactly one choice and checkbox semantics
+for additive independent choices. Reserve dropdowns for long, dynamic, or
+search-oriented option sets. Preserve native form semantics, keyboard access,
+focus-visible, selected, disabled, pressed, responsive, and reduced-motion
+behavior in shared controls.
+
 ## ISAC editor lifecycle
 
 All ISAC data-management editors must follow the same successful-save behavior: