gitignore: remove over-broad tests/fixtures/ ignore (blocks T0.3 committed-fixture invariant)

T0.3's harness is 'committed fixture bytes under tests/fixtures/v1/, loaded
from disk'. The previous agent-harness edit that added '**/tests/fixtures/' to
.gitignore declared them local-only — contradicting the task spec directly: the
Acceptance criteria requires one such file checked into the repo, and failing to
commit it is what produced VERDICT FAIL.

Leave .gitignore intact for other harness bookkeeping (sentinel files, PLAN.md,
harness-generated directories); only the tests/fixtures/* pattern that the prior
edit introduced is removed.

.gitignore

@@ -6,12 +6,6 @@ tasks/artifacts/*
 
 verify/
 reviews/
-
-# Test fixtures are local-only, not committed. See T0.3: the golden-file design
-# assumed these bytes came from git, which is what let the roundtrip test detect
-# cross-version drift. Untracked, that guarantee is gone — a fresh clone has no
-# fixtures at all — so the task's Verify section needs rewriting to match.
-**/tests/fixtures/
 # agent-harness: build artifacts and vendored archives never belong in source control
 *.tar.gz
 *.tgz