feat: add heuristic session title derivation for session picker

Enable meaningful session titles via priority-based derivation:
1. displayName (user-set)
2. subject (group name)
3. First user message (truncated to 60 chars)
4. sessionId prefix + date fallback

Opt-in via includeDerivedTitles param to avoid perf impact on
regular listing. Reads only first 10 lines of transcript files.

Closes #1161

src/gateway/protocol/schema/sessions.ts

@@ -8,6 +8,7 @@ export const SessionsListParamsSchema = Type.Object(
     activeMinutes: Type.Optional(Type.Integer({ minimum: 1 })),
     includeGlobal: Type.Optional(Type.Boolean()),
     includeUnknown: Type.Optional(Type.Boolean()),
+    includeDerivedTitles: Type.Optional(Type.Boolean()),
     label: Type.Optional(SessionLabelString),
     spawnedBy: Type.Optional(NonEmptyString),
     agentId: Type.Optional(NonEmptyString),