session-fail

When a turn ends not because the agent declared but because the API itself failed, this hook structurally marks the session error. A failed turn is a real outcome the board must show, and without this signal the session would freeze under whatever state it last held — reading as "active" or "awaiting" long after it actually died.

It is non-blocking and unconditional on the failure event: the failure already happened, so the only job is to record it truthfully. As a board-lifecycle hook it acts only on a GOVERNED session — resolved in the global store from the payload's session_id — and writes via spex session fail --session <id>. By turning an API error into a declared error state it keeps the stop-gate family's invariant intact — a session's displayed state always reflects what is really true of its last turn — for the one stop path the agent cannot narrate itself.