session: record owning process and detect exited sessions

State.Owner stores the proclive.Identity captured at turn start. OwnerLiveness/OwnerExited report when an ACTIVE session's agent process has gone away (clean exit, crash, kill, terminal close, reboot) without a SessionStop hook firing, falling back to the StuckActiveThreshold timeout when liveness is Unknown (no owner, cross-host, unsupported platform).