fix(tests): pin skill-audit digest fixtures to LF so the vectors hold on Windows

content_digest hashes fixture bytes raw, with no line-ending normalization,
because it covers arbitrary content and not just text. The fixture tree carried
no .gitattributes pin, so a Windows checkout with core.autocrlf=true rewrote
tree/SKILL.md, tree/tools.py and tree/scripts/run.py to CRLF and changed the
hash — test_content_digest_of_the_fixture_tree_matches_the_vector and
test_content_digest_ignores_a_build_cache failed on Windows only while CI stayed
green on Linux.

-text on the fixture directory stops conversion in both directions, so the LF
files stay LF and skill_md_crlf.txt keeps the CRLF bytes it deliberately tests.

Author: Ovtcharov

.gitattributes

@@ -2,6 +2,10 @@
 cpp/tests/fixtures/*.png binary
 cpp/tests/fixtures/*.jpg binary
 
+# content_digest hashes these fixture bytes raw (#2468), so an autocrlf checkout
+# would change the hash and fail the vectors on Windows only.
+tests/fixtures/skill_audit_digest/** -text
+
 # Every email-agent PR appends an [Unreleased] bullet to this changelog;
 # without a union merge, sibling PRs merged close together conflict on the
 # same insertion point.