Open Alert Investigation

Get-or-create the alert's investigation conversation (3c).

The mail/inbox deep link lands here: a focused follow-up thread for THIS alert (and the sibling checks its details reference via <CHECK:id>), distinct from the mixed-topic review conversation. Idempotent — re-opening the same alert returns the same conversation, so a refresh or a second click on the email never spawns a new thread.

Gated on read access (like get_alert) — except on governed databases, where it is manager-only: the conversation-load grant (user_can_use_alert_conversation) restricts governed threads to managers, so letting a non-manager create/own the alert's single shared thread would strand every later reader (and seed the thread with redacted context). Matches the review-conversation rule.