docs(plan): drop core:window:allow-set-opacity

Not a real Tauri 2.11.x permission identifier. Discovered when cargo
check listed the actual valid permissions. Opacity is Rust-side only
and routes through our custom command.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/superpowers/plans/2026-05-08-browserlay-mvp.md

@@ -1722,7 +1722,6 @@ Open `browserlay/src-tauri/capabilities/default.json` and replace its contents w
   "permissions": [
     "core:default",
     "core:webview:allow-create-webview-window",
-    "core:window:allow-set-opacity",
     "core:window:allow-set-always-on-top",
     "core:window:allow-set-ignore-cursor-events",
     "core:window:allow-close",
@@ -1736,6 +1735,8 @@ Open `browserlay/src-tauri/capabilities/default.json` and replace its contents w
 }
 ```
 
+> Note: `core:window:allow-set-opacity` is NOT a valid Tauri 2.11.x permission identifier — opacity is only exposed on the Rust `WebviewWindow`, not via Tauri's IPC ACL. Our custom `set_window_opacity` command (Task 16) doesn't need a per-command capability since `core:default` includes invocation of arbitrary app commands.
+
 - [ ] **Step 2: Create the overlay capability**
 
 Create `browserlay/src-tauri/capabilities/overlay.json` with: