feat(cloud-agent): improved sse keepalive handling

[tatoalo]

Problem

Cloud task watchers were treating keepalive frames like normal payloads, and the renderer could reuse preloaded local session state when a cloud run attached, which led to noisy reconnect behavior and duplicate startup history in cloud sessions.

Changes

• ignore SSE keepalive events in the cloud task watcher while still preserving reconnect logic
• skip the local reconnect path for cloud runs and reset same-run preloaded session history before the first cloud snapshot is applied
• added regression coverage