bug: queue status reports the newest slot as the "current execution", not the oldest #3319

Summary

services/slot_service.py::acquire_slot gives each new slot the number count + 1, so numbers get reused after out-of-order releases. services/capacity_manager.py::get_status sorts by that number and takes the first entry, but its comment promises "the oldest active slot".

Repro: cap 3. Acquire A, B and C; release A and B; acquire D. The status shows D as current instead of C.

Context

This is display only: GET /api/agents/{name}/queue. Monitoring reads only is_busy, so it isn't affected. Found by the 2026-10-06 /edge-cases sweep.

Acceptance Criteria

• get_status picks the slot with the smallest start score, not the lowest slot number. Alternatively, slot numbers become unique.
• tests/unit/test_ec_capacity_slots_edges.py::test_r56_get_status_current_execution_is_the_oldest_slot passes with its marker removed.