Keep right-canvas surface rail docked

Route right-canvas rail clicks through the explicit canvas opener so Browser, Editor, Files, and Desktop open in the canvas even after a modal session. Preserve modal-header surface switches as floating modal entry points, and document/test the entry-point split.

webui/components/canvas/AGENTS.md

@@ -14,6 +14,7 @@
 
 - Keep registered surfaces compatible with WebUI extension hooks.
 - Preserve responsive layout and avoid overlapping the chat/sidebar shells.
+- Right-canvas rail and tab buttons are explicit canvas entry points; they must open surfaces in the canvas even when the same surface was last used as a floating modal.
 
 ## Work Guidance
 

webui/components/canvas/right-canvas.html

@@ -46,7 +46,7 @@
               :class="{ 'is-active': $store.rightCanvas.isSurfaceActive(surface.id) && $store.rightCanvas.isOpen }"
               :title="surface.title"
               :aria-label="surface.title"
-              @click="$store.rightCanvas.openLatest(surface.id)"
+              @click="$store.rightCanvas.open(surface.id)"
             >
               <template x-if="surface.image">
                 <img class="right-canvas-surface-image" :src="surface.image" alt="" aria-hidden="true">