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>

CHANGELOG.md

@@ -46,6 +46,10 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- The markdown reader opens files given an absolute path again (e.g. a skill's
+  `SKILL.md` from the Claude Config tab). `resolveUnderRoot` no longer doubles an
+  absolute path onto the workspace root; absolute-under-root resolves, while
+  paths outside the root are still rejected. (T-194)
 - The composer slash typeahead now lists clide-owned commands — `/resume` and
   `/fork` (and `/clear`) surface even though the CLI probe doesn't advertise
   them, unioned onto whatever command source the composer uses. (T-162)