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

monitor-lets-go.example.yaml

@@ -26,6 +26,9 @@ restore_on_exit: true
 # External monitors that trigger docked mode.
 #   Plain name: matches the connector name (e.g. DP-1, HDMI-A-1).
 #   desc: prefix: matches by monitor description (survives rename).
+# Optional — if omitted or left empty, the daemon auto-detects external
+# monitors: any display whose connector is not eDP-/LVDS-/DSI- is treated
+# as external.
 external:
   - DP-1
   - DP-2
@@ -33,10 +36,10 @@ external:
 
 # Monitor layouts for each mode.
 # "portable" and "docked" are required.
-# Each mode lists monitors with their desired configuration.
+# Monitors connected but not listed in the mode are automatically disabled.
 #
 # Fields:
-#   name     — connector name or desc:description
+#   name     — connector name, desc:description, size:WxH, or size:WxH@R
 #   enabled  — true to show, false to disable
 #   mode     — "preferred" (auto-detect), "1920x1080@60", etc.
 #   position — "auto", "0x0", "1920x0", etc.
@@ -70,6 +73,11 @@ modes:
         mode: "3840x2160@60"
         position: "0x0"
         scale: 1.5
+      - name: size:1920x1080@60  # resolve by resolution + refresh rate
+        enabled: true
+        mode: preferred
+        position: auto
+        scale: 1.0
 
 # Shell commands run after a layout change.
 # Commands run concurrently; failures are logged but never crash the daemon.