Add ability to attach rules as context (#29109)

Release Notes:

- agent: Added support for adding rules as context.

crates/agent/src/ui/context_pill.rs

@@ -354,6 +354,16 @@ impl AddedContext {
                     .read(cx)
                     .is_generating_detailed_summary(),
             },
+
+            AssistantContext::Rules(user_rules_context) => AddedContext {
+                id: user_rules_context.id,
+                kind: ContextKind::Rules,
+                name: user_rules_context.title.clone(),
+                parent: None,
+                tooltip: None,
+                icon_path: None,
+                summarizing: false,
+            },
         }
     }
 }