Refactor: Extract session relationship population into shared helper

[samkeen]

Background

The fix for session/sub-agent relationships (populating parentSessionId and subAgentIds) introduced repeated code in 4 places:

• get_project()
• list_sessions()
• get_session()
• get_activity()

Proposal

Extract the relationship population logic into a shared helper function:

def populate_session_relationships(sessions: list[dict], project_dir: Path) -> None:
    """Populate parentSessionId and subAgentIds on session dicts in-place."""
    ...

This would reduce code duplication and make future changes easier to maintain.

Files to modify

• api/src/utils.py - Add helper function
• api/src/routes/projects.py - Use helper in 3 places
• api/src/routes/activity.py - Use helper in 1 place