docker: say where the runtime's event history begins, so a short history never reads as a quiet one (hq issue 348)

docker_events asked for three hours across the 2026-10-09 resolver outage on the control node answered
zero events: the runtime keeps only its last ~256 events in memory, and health-check execs filled them
with the last ten minutes. The answer now carries held, oldest_held and complete, and a history line
when the window reaches further back than what the runtime still holds. The type filter moves into the
tool, since the history's start is only known from all of it.

modules/docker/cmd/docker-tools/main.go

@@ -262,7 +262,8 @@ func tools(c *Client) []stdio.Tool {
 		{
 			Name: "docker_events",
 			Description: "What the runtime did in a window ending now (default the last 60 minutes, at most 24 hours): containers created, started, died, health changes, images pulled — with mesh_held. Exec events are left out unless asked; " +
-				"an exec's command line is shown with any secret it carried as [redacted: <what it was>] — a value of the container's environment named like a secret, a password in a URI, the word after a password flag.",
+				"an exec's command line is shown with any secret it carried as [redacted: <what it was>] — a value of the container's environment named like a secret, a password in a URI, the word after a password flag. " +
+				"The runtime keeps only its last ~256 events in memory, which health-check execs fill in minutes: complete=false and history say where what it holds begins, and an empty list then means gone, not quiet.",
 			Input: map[string]any{
 				"minutes": map[string]any{"type": "integer", "description": "how far back (default 60, at most 1440)"},
 				"type":    map[string]any{"type": "string", "description": "only one kind: container, image, network, volume, daemon, plugin or builder"},