fix: include external memory in sandbox OOM detection

[ntegrals]

Summary

The sandbox's OOM detection only checked RSS memory, missing external memory (native addon allocations, Buffers, graphics memory from Playwright/Chromium). For long-running agents, external memory can grow unbounded without triggering OOM — the process gets killed by the OS instead of gracefully shutting down.

Changes

OOM detection — now checks RSS + external against the limit instead of RSS alone. This catches memory growth from:

• Playwright's CDP protocol buffers
• Screenshot data (sharp image processing)
• Chromium's internal allocations exposed via process.memoryUsage().external

New metric — peakExternalMB added to SandboxMetrics so callers can see the native memory breakdown:

const result = await sandbox.run({ task, model });
console.log(result.metrics.peakMemoryMB);    // RSS peak
console.log(result.metrics.peakExternalMB);  // External/native peak

Files changed

File	Change
sandbox/src/sandbox.ts	Track peakExternalMB, check rss + external for OOM
sandbox/src/types.ts	Add peakExternalMB to SandboxMetrics

Test plan

• bun run build — all 3 packages compile clean
• bun run test — all 364 tests pass