[omartelo]

Summary

A merge on the Pulls screen already told the live provider sessions in the merged checkout. The session that opened one of them (its OriginSessionID) handed that work out and keeps notes on it too, but heard nothing, so the user had to tell it by hand.

• AnnounceMerge now builds its audience in mergeAudience: the live provider sessions in the checkout, then the live provider session that opened each of them, wherever it runs, each told once.
• An opener hears it even when the session it opened is parked: the work it handed out is done either way.
• Same notice text, so no catalog changes; delivery goes through deliver, mod or typed, for every provider.
• Only the direct opener hears it, not the session that opened the opener. Written down in docs/ceilings.md.

Test plan

• TestAnnounceMergeTellsTheSessionThatOpenedOneThere: opener in another project hears it once for two sessions it opened there, a nested opener inside the checkout hears it once, a bystander hears nothing. Fails without the change.
• go test -count=3 ./internal/relay/, go vet, gofmt clean.