fix: chat session reliability, cost, and hygiene (empty-response guard, tool truncation, delete, titles)

- Empty/refusal responses retried once, then surfaced as errors instead of silent blanks
- Chinese refusal boilerplate detected via denylist + non-ASCII heuristic
- Bulk-tool preference added to SOUL.md (list_lxcs over per-entity get_lxc_state)
- Tool results truncated to 4KB on persist; get_state_snapshot filters null-state entities
- Session delete (DELETE /sessions/{id} + confirm-on-second-click UI)
- Session titles auto-generated from assistant answer instead of raw user message

web/src/lib/api.ts

@@ -31,6 +31,11 @@ export async function fetchMessages(sessionId: string): Promise<Message[]> {
   return data.messages ?? []
 }
 
+export async function deleteSession(sessionId: string): Promise<boolean> {
+  const res = await fetch(`${BASE}/sessions/${sessionId}`, { method: 'DELETE' })
+  return res.ok
+}
+
 export interface ChatEvent {
   type: string
   data: any