fix markdown reader 404 on absolute paths

resolveUnderRoot joined an absolute input onto the workspace root
(/repo + /repo/x → /repo/repo/x), so files.read 404'd on a file that
exists. The Claude Config tab hands the reader a skill's absolute
SKILL.md path, which hit this. Normalize an absolute input as-is; the
existing containment check still rejects absolute paths outside the
root, so the T-102 boundary is preserved.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/src/files/path_safety.dart

@@ -26,7 +26,12 @@ class PathOutsideRoot implements Exception {
 /// without touching disk (T-102).
 String resolveUnderRoot(Directory root, String relative) {
   final rootPath = _normalize(root.absolute.path);
-  final joined = _normalize('$rootPath${Platform.pathSeparator}$relative');
+  // An absolute input is normalized as-is rather than joined onto the
+  // root — otherwise a path already under the root gets doubled
+  // (`/repo` + `/repo/x` → `/repo/repo/x`) and resolves to nothing.
+  // Containment is still enforced below, so an absolute path *outside*
+  // the root is rejected exactly as a `..` traversal is.
+  final joined = relative.startsWith(Platform.pathSeparator) ? _normalize(relative) : _normalize('$rootPath${Platform.pathSeparator}$relative');
 
   // Containment check: joined must equal rootPath, or start with
   // rootPath + separator. Equality covers `relative == ''` (the