Return active file locks grouped by agent for the dashboard.

Reads the persisted lock state from .sdd/runtime/file_locks.json and returns it in a dashboard-friendly format with both a flat list and an agent-grouped view.

Returns: JSON with all_locks (flat list sorted by path), locks_by_agent (dict keyed by agent_id with files list + task info + elapsed_s), count (total lock count), and ts (generation timestamp).