agent-os: sandbox import hook modifies global sys.meta_path

## Problem
`SandboxImportHook.install()` inserts into `sys.meta_path` which is process-global. In multi-agent scenarios, one agent's sandbox affects all others.

**File:** `src/agent_os/sandbox.py` (line 108)

## Impact
Thread-safety issues; one agent's uninstall removes another's hook.

## Suggested Fix
Use per-agent import contexts or thread-local meta_path patching.