Transcript hydration now tracks message origin for artifact-state reconstruction

What

• Transcript user/assistant messages can now be tagged remoteSourced: true, and the function that rebuilds artifact state from history now tracks which tool calls came from a remote source and skips local-only reconciliation for those.
• A new check decides whether to rebuild artifact state from local history at all, based on whether a message's origin is "local" and it is seedEligible.
• The call that seeds/reconciles initial transcript state now passes an origin (defaulting to "local") and a seedEligible flag, threaded through a new initialMessagesOrigin prop on the session engine constructor.

Why This distinguishes messages that came from a remote source (such as headless or remote sessions) from local ones, so artifact-state reconstruction only reconciles local history instead of double-processing remote-sourced tool calls.