Fix file tree clicks and redesign add-file UI on instructions tab

- Add onClick handler to file row div in PackageFileTree so clicks
  anywhere on the row select the file (not just the inner button)
- Replace "Add" button with compact "+" icon that reveals an inline
  input with Create/Cancel actions
- Hide file name input until "+" is clicked to reduce visual clutter
- Validate new file paths: reject ".." path traversal segments
- Change placeholder from "docs/TOOLS.md" to "TOOLS.md"

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/components/PackageFileTree.tsx

@@ -286,6 +286,7 @@ export function PackageFileTree({
             style={{
               paddingInlineStart: `${TREE_BASE_INDENT + depth * TREE_STEP_INDENT - 8}px`,
             }}
+            onClick={() => onSelectFile(node.path)}
           >
             {showCheckboxes && (
               <label className="flex items-center pl-2">