Slack agent flow (create_agent → provisioned Slack bot)

Composes the Slack extension skills into one conversational flow: a user tells an existing Slack-wired agent "create an agent called Research", and the new agent doesn't just exist as a send_message destination — it arrives in Slack as its own bot. The host provisions a Slack app for it (managed broker, or a workspace manager token), registers it as a slack-<name> instance, hot-starts the adapter in the running host, opens a DM between the new bot and the operator, opens a three-way MPIM (operator + originating bot + new bot) registered as an agent-to-agent room, and wires everything so both agents hear the room. The flow also adds two agent-facing room actions — create_room (one shared room with N agents at once, the team primitive) and add_to_room (grow a room by one agent) — and extends the base create_agent tool with the flow's purpose / allow_guests / room parameters. Non-Slack sessions are untouched: create_agent from any other channel behaves exactly as upstream.