[hobzcalvin]

Adds create_inbox_items, the Notification Platform entrypoint for the Universal Inbox (ISWF-3737), and has every existing inbox producer call it directly. The contract is being discussed on ISWF-3725.

def create_inbox_items[SubjectT, DataT: InboxEventData](
    *,
    event_type: InboxEventType[SubjectT, DataT],
    subject: SubjectT,
    data: DataT,
    recipients: Iterable[Actor],
    actor: InboxActor,
) -> None:

Recording. recipients takes users and teams. Teams expand to their active members, recipients outside the subject's organization are dropped, and a user reached both directly and through a team gets one event. Recording goes through InboxEventType.record(), which still drops the actor and now returns the events it created.

One way in. Assignment (GroupAssignee.objects.assign), comment mentions (issues/comments.py), Seer PR-ready (seer/entrypoints/operator.py), and investigation completion (orchestration_events.py) now call create_inbox_items with users and teams. The record_issue_* and record_notebook_completed wrappers are removed.

Notifications (skeleton). INBOX_NOTIFIERS in notifications/platform/inbox.py maps event types to notifiers; the inbox types don't know about notifications. Behind organizations:inbox-notifications, the users who got a new event are handed to the event type's notifier after the transaction commits, so a retry that records nothing notifies nobody. Each recipient is an individual user, notified per their personal settings; teams get no channel post.

• The mention event types have a notifier: it loads the note's Activity and resolves each user's targets with UserRoutingStrategy and the WORKFLOW setting. Building the notification from the existing activity template and sending it is a TODO.
• Other event types record without notifying. Assignment can use the same notifier once its data carries activity_id (feat(inbox): Add durable occurrence IDs to event data #130397).

Not in this PR:

• Building and sending the notifications, reusing the existing activity notifications.
• Per-event-type settings, e.g. "assigned to your team" vs "assigned to you" (ISWF-3670).
• The workflow engine's ADD_TO_INBOX action (ISWF-3710). The existing activity notifications for these events keep sending, so cutover has to avoid double-sending.

[linear-code]

ISWF-3737