Show turn-scoped Slack footer on event replies

[sentry-junior]

Event-triggered Slack replies still look like orphan assistant messages. Humans do not see why Junior posted, or how many updates that reply covered.

Current behavior

• Resource-event turns enqueue as mailbox input and run as the system resource-event actor.
• Agent-facing prompt framing is handled separately (see Frame resource-event turns for humans and agents #1638 / fix(resource-events): Frame event turns for humans and agents #1639).
• Slack only shows the later assistant reply. Destination-visible replyAttribution footer chrome exists for other turn kinds (dispatch, scheduled tasks) via sendSlackReply, but resource-event turns do not attach it.
• A turn can include the primary event plus messages grouped at start and messages live-steered while the turn runs. Those inputs all belong to one turn.

Gap

• Status updates and confirmations from event turns lack trigger context in Slack (resource, what changed, multi-update count).
• Footer chrome needs to represent the whole turn, not only the primary mailbox row: initially grouped inputs and live-steered inputs during the turn still count toward that turn’s chrome.

Related: #1638 (agent-facing event prompt). Human-visible footer chrome was pulled out of #1639 to keep that PR prompt-only.

Proposal (from product discussion)

• Add destination-visible footer chrome for resource-event replies (for example Update · <summary> / N updates · <latest> (+N-1 more)).
• Scope the footer to the entire turn: map the turn’s contributing messages (grouped batch + live-steered mid-turn inputs) into one footer.
• Prefer a simple map of messages already on the turn over a separate durability side channel.

Requested by David Cramer.

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