Function add_tools 

pub(crate) fn add_tools(
    builder: McpServerBuilder,
    facade: Arc<dyn SlackFacade>,
    slot: SessionSlot,
) -> McpServerBuilder

Attach the Slack tools to an existing McpServerBuilder. Called for BOTH lead and worker sessions (any-caller), so build_forge_server invokes this unconditionally. subscribe / unsubscribe are scoped to the caller’s own records, so they need the caller slot.