fix(gitignore): cover test_hierarchical_test fixture READMEs — stop scan-all dirty tree (#183) (#184)

a8f1732 deleted the 4 committed README_AI.md files under
tests/legacy/test_hierarchical_test/ to stop the pytest rmtree-rebuild
dirty-tree path, but missed .gitignore — scan-all (root config includes
tests/) regenerated them, dirtying the tree every run. Sibling gap of
#135 (char_graphbuffer), which lives under tests/legacy/ so #135's rule
missed it.

Added tests/legacy/test_hierarchical_test/**/README_AI.md to .gitignore
+ test_gitignore_fixtures.py pinning both rules (#183 + #135) and the
cli_parse/graph_export carve-out (stay committable).

Verified: TDD red (disabled rule → test fails), green after, full suite
1803 passed, scan-all→pytest cycle leaves 0 hierarchical dirty files.