fix(fleet): compose system settings from the installed flattened home layout

The installed ~/.config/mosaic home flattens the repo's
packages/mosaic/framework/ prefix: the real file is
<home>/runtime/<harness>/settings.json, exactly as launch.ts already
resolves it everywhere. The fleet launch composition leaked the repo
layout (framework/runtime/...) into the system layer path, so a real
installed home failed with COMPOSITION_FAILED while the temp-fixture
specs (which mirrored the same wrong prefix) stayed green. Found by the
in-box hour-gate dry-run against the installed mos-dev-stage home.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Dtdjx4Gxude9fwyLezCrhh

packages/mosaic/src/commands/fleet-launch-command.ts

@@ -577,7 +577,7 @@ export function resolveFleetLaunchComposition(
   const layers: SettingsLayer[] = [
     readSettingsLayer(
       'system',
-      join(roots.systemHome, 'framework', 'runtime', profile.harness, 'settings.json'),
+      join(roots.systemHome, 'runtime', profile.harness, 'settings.json'),
       true,
     ),
     readSettingsLayer(