Perf: Optimize get_parent_session_id() to read only first line

[samkeen]

Current behavior

get_parent_session_id() in api/src/utils.py reads the entire JSONL file to extract the parent session ID:

content = agent_path.read_text()
lines = parse_jsonl_file(content)
if lines:
    return lines[0].get("sessionId")

Problem

For large sub-agent sessions with many messages, this reads unnecessary data. Only the first line is needed.

Proposed fix

with open(agent_path) as f:
    first_line = f.readline()
    if first_line:
        return json.loads(first_line).get("sessionId")

This would be more efficient for projects with many or large sub-agent files.