feat: mkdir command creates folders via _index.md

Uses a dedicated /api/cli/folders endpoint (separate from notes because
_index slugs contain underscores which the notes slug validator rejects).
Derives a readable title from the slug, navigates into the new folder
immediately even before any notes exist.

NozCli/Commands/MkdirCommand.cs

@@ -0,0 +1,80 @@
+using NozCli.Core.Security;
+using NozCli.Repl;
+using Spectre.Console;
+
+namespace NozCli.Commands;
+
+public class MkdirCommand : ICommand
+{
+    public string Name        => "mkdir";
+    public string Description => "Create a new folder (writes _index.md)";
+    public string Usage       => "mkdir <folder-name>";
+
+    public async Task ExecuteAsync(string[] args, ReplContext ctx, CancellationToken ct = default)
+    {
+        var folderArg = args.FirstOrDefault();
+        if (folderArg is null)
+        {
+            AnsiConsole.MarkupLine("[red]Usage:[/] mkdir <folder-name>");
+            return;
+        }
+
+        // Resolve full folder path relative to current navigation
+        var folderPath = ResolveFolderPath(folderArg.TrimEnd('/'), ctx);
+
+        if (!SlugGuard.IsValid(folderPath))
+        {
+            AnsiConsole.MarkupLine("[red]Ungültiger Ordnername.[/] Nur Kleinbuchstaben, Ziffern und Bindestriche erlaubt.");
+            return;
+        }
+
+        // Derive a readable title from the last segment
+        var lastSegment = folderPath.Split('/')[^1];
+        var title       = ToTitle(lastSegment);
+
+        await AnsiConsole.Status().StartAsync($"Erstelle [cyan]{folderPath}[/]…", async _ =>
+            await ctx.Api.PutFolderIndexAsync(folderPath, title, ct: ct));
+
+        // Navigate into the new folder
+        NavigateInto(folderPath, ctx);
+
+        AnsiConsole.MarkupLine($"[green]✓[/] Ordner erstellt → {ctx.CurrentPath}");
+        AnsiConsole.MarkupLine($"[dim]Erstelle deine erste Notiz mit [white]new <name>[/][/]");
+    }
+
+    private static string ResolveFolderPath(string segment, ReplContext ctx)
+    {
+        // Already a full multi-segment path (user typed full slug)
+        if (segment.Contains('/')) return segment;
+
+        if (ctx.CurrentTopic is null)
+            return segment;
+
+        if (ctx.CurrentSubPath is null)
+            return $"{ctx.CurrentTopic}/{segment}";
+
+        return $"{ctx.CurrentTopic}/{ctx.CurrentSubPath}/{segment}";
+    }
+
+    private static void NavigateInto(string folderPath, ReplContext ctx)
+    {
+        var parts = folderPath.Split('/');
+        if (parts.Length == 1)
+        {
+            ctx.CurrentTopic   = parts[0];
+            ctx.CurrentSubPath = null;
+        }
+        else
+        {
+            ctx.CurrentTopic   = parts[0];
+            ctx.CurrentSubPath = string.Join('/', parts[1..]);
+        }
+    }
+
+    // "new-subfolder" → "New Subfolder"
+    private static string ToTitle(string slug) =>
+        string.Join(' ', slug.Split('-').Select(Capitalize));
+
+    private static string Capitalize(string s) =>
+        s.Length == 0 ? s : char.ToUpperInvariant(s[0]) + s[1..];
+}