fix(docs): remove stale edge names from data_model.md

What does this MR do and why?

Removes four stale edge names from docs/design-documents/data_model.md that do not exist in the current ontology, and corrects the Mermaid diagram and relationship table to reflect actual implemented edges.

Stale edges removed: COMMENTS_ON, IS_COMMENT_ON, HAS_MERGE_REQUEST, HAS_VULNERABILITY. Additional corrections: HAS_LABEL updated to include MergeRequest; CONTAINS, IN_GROUP, IN_MILESTONE, AUTHORED, CLOSED, HAS_NOTE, HAS_IDENTIFIER, TRIGGERED rows corrected with missing variants; nine edges absent from docs added (MEMBER_OF, CREATOR, OWNER, ASSIGNED, FIXES, RELATED_TO, FROM_BRANCH, SCANS, RAN_BY). Opening example sentence updated from COMMENTS_ON to AUTHORED. Mermaid diagram updated to match.

Related Issues:

Closes #724 (closed)

Testing:

mise lint:docs passes (markdownlint, Vale, lychee all clean on data_model.md).

Performance Analysis:

Documentation-only change. No performance regression.