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.ts

@@ -2076,7 +2076,10 @@ export function registerFleetCommand(program: Command, deps: FleetCommandDeps =
   // `fleet agent new` owns user-data harness homes under ~/.mosaic. The
   // existing roster-v2 CRUD remains direct fleet control-plane CRUD, so there
   // is one `agent` namespace but deliberately separate state authorities.
-  registerFleetAgentScaffoldCommand(cmd, { fleetDataHome: deps.fleetDataHome });
+  registerFleetAgentScaffoldCommand(cmd, {
+    ...(deps.fleetDataHome === undefined ? {} : { fleetDataHome: deps.fleetDataHome }),
+    mosaicHomeFor: () => cmd.opts<{ mosaicHome: string }>().mosaicHome,
+  });
   // Roster-v2 desired-state mutations belong directly to the fleet control
   // plane; they do not share the root `mosaic agent` gateway-backed surface.
   registerFleetAgentCrudCommands(cmd, deps);