tool: separate a tool that is off from a backend that is down

An enabled MCP or smarthome row with no backend returned ErrNotEnabled,
and actionAct reads ErrNotEnabled as "this is unknown, draft a
proposal". So a tool Kami had already approved, whose server happened to
be restarting, produced a second proposal row and an answer saying the
tool needs approval. The right answer is that the server is down.
ErrNotConnected carries that, and the act path maps it, ErrNoServer and
ErrToolGone to replies that say which of the three happened.

Found in review of #71.

cmd/mavend/actions_act.go

@@ -53,6 +53,13 @@ func (h *reactiveHandler) actionAct(ctx context.Context, dec router.Decision) st
 			return "выполнить «" + phrase + "»? скажи «да» или «нет»."
 		case errors.Is(err, tool.ErrNotEnabled):
 			return h.proposeGap(ctx, dec)
+		case errors.Is(err, tool.ErrNotConnected), errors.Is(err, mcp.ErrNotConnected), errors.Is(err, mcp.ErrNoServer):
+			// The row is enabled and the backend is gone. Drafting a proposal
+			// for it (the ErrNotEnabled path) would be answering the wrong
+			// question.
+			return "этот инструмент включён, но сервер, который его выполняет, сейчас не подключён."
+		case errors.Is(err, mcp.ErrToolGone):
+			return "сервер больше не предлагает этот инструмент — я сняла его с разрешённых, посмотри на /tools."
 		case errors.Is(err, mcp.ErrNeedsArgs):
 			// An MCP tool that wants named arguments a spoken verb cannot
 			// supply. Guessing them would be a wrong act, so she says so