Live signal stream for one connection's agents (SSE)

The same protocol as /v1/otel/stream — Last-Event-ID resume, event: gap on a stale cursor or a lagged subscriber — filtered to signals whose agent is one of the connection's. Signals with no agent (a human's own dashboard actions) are never emitted here. The agent set is fixed at connect time; an agent added later appears on the next connect.

Stream slots are counted per platform app (five), not per connection.

A plt_ key sees one connection's agents, never the end-user's org: that org may hold agents from other platforms or the user's own. The set is agents attributed to the connection plus the ones listed on it, constrained to the connection's org. A connection that does not belong to the calling app is a 404.