fix(desktop): use null-coalescing fallback in AppShell composite reconcile key

Without the '?? "none"' fallback, a null activeCommunity?.id produces
the string "undefined-N" rather than "none-N". Both are valid change
signals, but the explicit fallback matches the existing pattern at
line 233 of the same file and satisfies Thufir's pass-2 finding C7.

The line expands to 3 lines after biome formatting (88 chars), landing
AppShell.tsx at exactly 1000 gate-counted lines — still within the
1000-line ratchet (gate condition is > 1000).

Co-authored-by: Will Pfleger <pfleger.will@gmail.com>
Signed-off-by: Will Pfleger <pfleger.will@gmail.com>

desktop/src/app/AppShell.tsx

@@ -124,7 +124,9 @@ export function AppShell() {
   const mainInsetRef = React.useRef<HTMLElement>(null);
   const location = useLocation();
   const queryClient = useQueryClient();
-  useManagedAgentRuntimeReconciliation(`${activeCommunity?.id}-${reinitKey}`);
+  useManagedAgentRuntimeReconciliation(
+    `${activeCommunity?.id ?? "none"}-${reinitKey}`,
+  );
   const {
     goAgents,
     goChannel,