fix(gitignore): scope runtime data ignores

[200lz]

Summary

• Anchor the repository-root runtime dataset ignore rules.
• Preserve the MMEB-v3 runtime data exclusions.
• Prevent unrelated nested directories named data or datasets from being silently ignored.

Rationale

Gitignore directory patterns without a leading slash match at any depth. The existing rules therefore also exclude nested source directories with the same names.

Validation

• Verified root data/ and datasets/ remain ignored.
• Verified mmeb_v3_eval/data/ and mmeb_v3_eval/datasets/ remain ignored.
• Verified nested package/data/ and package/datasets/ source paths are not ignored.
• Verified __pycache__/ remains ignored.
• Ran git diff --check.