Re-land upstream #1638 onto the modular structure. Adds the
slack_start_new_thread tool (posts a top-level Slack message and
dispatches a fresh agent run for a broken-out task via the durable
dispatch_agent_run contract), wires it into the agent tool list and
tools/__init__, adds prompt guidance, and excludes it from plan mode so
it can't bypass the approval flow. Tool imports only live modules.
Refs #80, upstream #1638
