feat(filesystem): add create-folder API for workspace picker

Adds a secure endpoint for creating a single subfolder in the current filesystem listing, and wires the non-native directory browser UI to create + enter the new folder.

packages/server/src/server/routes/filesystem.ts

@@ -11,6 +11,11 @@ const FilesystemQuerySchema = z.object({
   includeFiles: z.coerce.boolean().optional(),
 })
 
+const FilesystemCreateFolderSchema = z.object({
+  parentPath: z.string().optional(),
+  name: z.string(),
+})
+
 export function registerFilesystemRoutes(app: FastifyInstance, deps: RouteDeps) {
   app.get("/api/filesystem", async (request, reply) => {
     const query = FilesystemQuerySchema.parse(request.query ?? {})
@@ -24,4 +29,26 @@ export function registerFilesystemRoutes(app: FastifyInstance, deps: RouteDeps)
       return { error: (error as Error).message }
     }
   })
+
+  app.post("/api/filesystem/folders", async (request, reply) => {
+    const body = FilesystemCreateFolderSchema.parse(request.body ?? {})
+
+    try {
+      const created = deps.fileSystemBrowser.createFolder(body.parentPath, body.name)
+      reply.code(201)
+      return created
+    } catch (error) {
+      const err = error as NodeJS.ErrnoException
+      if (err?.code === "EEXIST") {
+        reply.code(409).type("text/plain").send("Folder already exists")
+        return
+      }
+      if (err?.code === "EACCES" || err?.code === "EPERM") {
+        reply.code(403).type("text/plain").send("Permission denied")
+        return
+      }
+
+      reply.code(400).type("text/plain").send((error as Error).message)
+    }
+  })
 }