feat: add fuzzy filter to TUI session picker

Users can now type to filter sessions in real-time:
- FilterableSelectList component wraps pi-tui's fuzzyFilter
- Matches against displayName, label, subject, sessionId
- j/k navigation, Enter selects, Escape clears filter then cancels
- Uses derivedTitle from previous commit for better display

Refs #1161

src/tui/gateway-chat.ts

@@ -64,6 +64,7 @@ export type GatewaySessionList = {
     lastProvider?: string;
     lastTo?: string;
     lastAccountId?: string;
+    derivedTitle?: string;
   }>;
 };
 
@@ -183,6 +184,7 @@ export class GatewayChatClient {
       activeMinutes: opts?.activeMinutes,
       includeGlobal: opts?.includeGlobal,
       includeUnknown: opts?.includeUnknown,
+      includeDerivedTitles: opts?.includeDerivedTitles,
       agentId: opts?.agentId,
     });
   }