fix(fleet): seed seat MCP preflight config

AMD1213-B5: derive Claude seat MCP configuration from the active installed runtime base and inspect the isolated seat during fleet launch.

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

@@ -110,6 +110,7 @@ export interface FleetLaunchComposition {
   readonly profilePath: string;
   readonly profile: FleetAgentLaunchProfile;
   readonly agentDir: string;
+  readonly systemHome: string;
   readonly seatHome: string;
   readonly settings: {
     readonly layers: readonly SettingsLayer[];
@@ -749,6 +750,7 @@ export function resolveFleetLaunchComposition(
   return {
     name,
     profilePath,
+    systemHome: roots.systemHome,
     profile,
     agentDir,
     seatHome,
@@ -939,7 +941,10 @@ export function registerFleetLaunchCommand(
         applyFleetLaunchComposition(plan);
         console.log(`[mosaic] bundle: ${plan.bundle.display}`);
         const launcher = deps.launcher ?? launchFleetRuntime;
-        launcher(plan.profile.harness, plan.argv.slice(1), plan.env, { agentDir: plan.agentDir });
+        launcher(plan.profile.harness, plan.argv.slice(1), plan.env, {
+          agentDir: plan.agentDir,
+          mosaicHome: plan.systemHome,
+        });
       } catch (error: unknown) {
         process.exitCode = 1;
         const code = error instanceof FleetLaunchError ? `${error.code}: ` : '';