docs(ontology): clarify old_path vs new_path on MergeRequestDiffFile

[skip schema-version-check]

What does this MR do and why?

LLM agents querying the graph default to new_path for file-based lookups because the old descriptions didn't indicate which field to prefer. old_path is stable across renames and is the better default.

• old_path description now says: "Preferred for file lookups."
• new_path description now says: "Differs from old_path only on renames."

Observed during a reviewer-recommendation query for a GitLab file. The agent chose new_path first and got zero results.

Related Issues

Relates to #603 (closed)

Testing

Ontology validation passes.

Performance Analysis

• This merge request does not introduce any performance regression. If a performance regression is expected, explain why.