fix(ontology): add commit_sha and position fields to source_code default_columns

What does this MR do and why?

Source code entities store commit_sha and position fields in ClickHouse, but queries without explicit column selection never return them — they aren't in default_columns, so the compiler omits them from the SELECT. Agents and CLI users see null for these fields and can't anchor symbols to commits or source locations without knowing to request columns explicitly.

Adds missing fields to default_columns across all source_code entities:

• Definition: commit_sha, start_line, end_line
• ImportedSymbol: commit_sha, start_line, end_line
• File: commit_sha
• Directory: commit_sha

No schema migration — data already exists in ClickHouse, just wasn't selected. default_columns only affects query-time column selection, not DDL.

[skip schema-version-check]

Related Issues

Closes gitlab-org/gitlab#600167

Testing

Existing ontology test updated to match new default columns.

Performance Analysis

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