Every Everruns user starts in a pinned Platform Chat thread

A new user used to land on Chats with an empty list and a counterpart picker. The app now creates a thread on the built-in Platform Chat harness on first entry, pins it, and ends onboarding in it. An existing thread is reused and an archived one is not recreated.

Platform Chat also runs on a wider capability set, adding current time, message timestamps, a to-do list, prompt caching, tool call repair and parallel tool calls. Existing organizations pick these up without a migration.

See Platform Chat. #3508, #3507