feat: scaffold codespace backend

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/fs/fileinfo.go

@@ -0,0 +1,13 @@
+package fs
+
+import "time"
+
+// FileInfo describes a file or directory within a workspace.
+// Path is always workspace-relative; host absolute paths are never exposed.
+type FileInfo struct {
+	Name    string    `json:"name"`
+	Path    string    `json:"path"`
+	IsDir   bool      `json:"isDir"`
+	Size    int64     `json:"size"`
+	ModTime time.Time `json:"modTime"`
+}