feat: add App.getAppGraph() for app-only Graph access

[heyitsaamir]

Summary

• App.getAppGraph(tenantId?) — new public method returning a GraphClient for app-only Graph operations. Accepts optional tenantId for multi-tenant support.
• Added /app-users and /app-users ctx commands to the graph example demonstrating both app.getAppGraph() and ctx.appGraph patterns.

// App-level Graph access, no ActivityContext needed
const graph = app.getAppGraph();                      // default tenant
const graph = app.getAppGraph("some-tenant-id");      // explicit tenant
const users = await graph.call(endpoints.users.list);