Make agent continuation and final delivery conversation-neutral

[dcramer]

Objective

Use the same continuation lifecycle for root conversations and subagents while allowing different final delivery targets.

Scope

• Extract Slack-specific continuation and completion behavior behind runtime ports.
• Keep mailbox, lease, session-log, timeout, auth-pause, and retry behavior shared.
• Support platform delivery and internal parent-conversation delivery without separate agent runtimes.

Completion

• Root and child conversations can use the same continuation endpoint and queue callback.
• A resumed run restores its durable execution profile and state.
• Slack delivery remains behavior-compatible.