feat: size: prefix, external auto-detection, auto-disable stale monitors

- size:WxH and size:WxH@R resolution for config names
- auto-detection of external monitors when external list is empty (any non-internal display)
- ensureCleanLayout: explicitly disable connected monitors not in target layout
- update docs (README, example.yaml) for all three features

internal/config/config.go

@@ -157,10 +157,8 @@ func (c *Config) validate() error {
 	if countEnabled(docked.Monitors) == 0 {
 		return errors.New("docked mode must have at least one enabled monitor")
 	}
-	// At least one external monitor must be listed.
-	if len(c.External) == 0 {
-		return errors.New("at least one external monitor must be specified")
-	}
+	// External list is optional. When empty, the daemon auto-detects
+	// external monitors (any non-internal display with non-zero dimensions).
 
 	return nil
 }
@@ -195,19 +193,40 @@ func countEnabled(entries []MonitorEntry) int {
 	return n
 }
 
-// MatchesExternal checks whether a monitor name or description matches any
-// entry in the External list. Supports two match modes:
+// isInternalConnector returns true if the connector name matches a known
+// internal display pattern (eDP, LVDS, DSI). These are always part of the
+// laptop or tablet and should never trigger docked mode.
+func isInternalConnector(name string) bool {
+	prefixes := []string{"eDP-", "LVDS-", "DSI-", "EDP-"}
+	for _, p := range prefixes {
+		if strings.HasPrefix(name, p) {
+			return true
+		}
+	}
+	return false
+}
+
+// MatchesExternal checks whether a monitor matches the External list.
+// Supports match modes:
 //
-//   - Plain name: exact match against MonitorEntry.Name
-//   - desc: prefix: substring match against MonitorEntry.Description
+//   - Plain name: exact match against name
+//   - desc: prefix: substring match against description
+//
+// When the External list is empty, any monitor that is not an internal
+// display connector (eDP-, LVDS-, DSI-) is automatically external.
 func (c *Config) MatchesExternal(name, description string) bool {
+	if len(c.External) == 0 {
+		return !isInternalConnector(name)
+	}
+
 	for _, ext := range c.External {
-		if strings.HasPrefix(ext, "desc:") {
+		switch {
+		case strings.HasPrefix(ext, "desc:"):
 			desc := strings.TrimPrefix(ext, "desc:")
 			if strings.Contains(description, desc) {
 				return true
 			}
-		} else {
+		default:
 			if name == ext {
 				return true
 			}