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>

test/daemon/files_commands_test.dart

@@ -97,6 +97,16 @@ void main() {
     expect(r.data['path'], 'README.md');
   });
 
+  test('files.read accepts an absolute path under the workspace root', () async {
+    // Regression: the markdown reader publishes absolute skill paths
+    // (e.g. .claude/skills/.../SKILL.md). An absolute path under root
+    // must resolve, not double onto the root and 404.
+    final abs = '${sandbox.absolute.path}/README.md';
+    final r = await call('files.read', {'path': abs});
+    expect(r.ok, isTrue);
+    expect(r.data['content'], 'hi');
+  });
+
   test('files.read without a path returns toolError', () async {
     final r = await call('files.read', const {});
     expect(r.ok, isFalse);