feat(10-02): add GraphDirectoryUser model and IGraphUserDirectoryService interface

- GraphDirectoryUser positional record with DisplayName, UPN, Mail, Department, JobTitle
- IGraphUserDirectoryService.GetUsersAsync with clientId, IProgress<int>?, CancellationToken
- Follows existing GraphUserSearchService namespace pattern

SharepointToolbox/Core/Models/GraphDirectoryUser.cs

@@ -0,0 +1,12 @@
+namespace SharepointToolbox.Core.Models;
+
+/// <summary>
+/// Represents a directory user returned by <see cref="SharepointToolbox.Services.IGraphUserDirectoryService"/>.
+/// Used by Phase 13's User Directory ViewModel to display and filter tenant members.
+/// </summary>
+public record GraphDirectoryUser(
+    string DisplayName,
+    string UserPrincipalName,
+    string? Mail,
+    string? Department,
+    string? JobTitle);

SharepointToolbox/Services/IGraphUserDirectoryService.cs

@@ -0,0 +1,26 @@
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Services;
+
+/// <summary>
+/// Enumerates all enabled member users from a tenant via Microsoft Graph.
+/// Used by Phase 13's User Directory ViewModel to populate the browse grid.
+/// </summary>
+public interface IGraphUserDirectoryService
+{
+    /// <summary>
+    /// Returns all enabled member users in the tenant associated with <paramref name="clientId"/>.
+    /// Iterates through all pages using the Graph SDK PageIterator until exhausted or cancelled.
+    /// </summary>
+    /// <param name="clientId">The client/tenant identifier used to obtain a Graph token.</param>
+    /// <param name="progress">
+    /// Optional progress reporter — receives the running count of users fetched so far.
+    /// Phase 13's ViewModel uses this to show "Loading... X users" feedback.
+    /// Pass <c>null</c> for no progress reporting.
+    /// </param>
+    /// <param name="ct">Cancellation token. Iteration stops when cancelled.</param>
+    Task<IReadOnlyList<GraphDirectoryUser>> GetUsersAsync(
+        string clientId,
+        IProgress<int>? progress = null,
+        CancellationToken ct = default);
+}