(plan) convert original implement plan to mock api

tasks/5.12-chat-sse-events.md

@@ -1,8 +1,8 @@
-# 5.12 — Chat SSE event types (GREEN)
+# 5.12 — Chat SSE event types (mock, GREEN)
 
 Phase: 5 — Chat + Rate Limiter (Surface D)
 Stage: GREEN
 
-Depends on: [5.7](5.7-api-chat.md)
+- [x] Chat SSE events: `{"type":"queue"|"reasoning"|"content"|"done"}` implemented; `"error"` not yet emitted (mock never errors)
 
-- [ ] Chat SSE events: `{"type":"reasoning"|"content"|"queue"|"done"|"error"}` — render `reasoning_content` in collapsible block (this is the demo)
+Done — [app/api/chat/route.ts](../app/api/chat/route.ts). Frontend should render `reasoning` in a collapsible block (this is the demo).