Resource leaks in long-running sessions

**Severity:** Low
**Component:** Core
**Simulation:** SIM-L2-005
**Sprint:** 5

## Description
WatchHandle and SyncSession lifecycle not properly managed may cause memory/FD leaks.

## Proposed Fix
Implement reference counting and periodic leak detection.

## Source
Imported from `.straymark/02-design/risk-analysis/BACKLOG-simulation-issues.md` on 2026-05-29.