agent: container memory.max ~1.93GB causes exit 143 on Node tooling

## Summary

Today (2026-05-28, ~21:30-21:55 UTC) several Opus 4.7 impl sessions stalled mid-task with `Agent error: Command failed with exit code 143` (SIGTERM). Sessions confirmed affected: `impl-af6f01dd`, `impl-2d3f4375`; likely also `impl-247fdb3b` + `impl-5f51ff89`.

## Root cause

Container cgroup `memory.max` was `2,066,325,504` bytes (~1.93 GB), below the 2-4 GB ceiling Node tooling (`jest`, `tsc`, `next build`) routinely uses. The OOM killer SIGTERMed the Node child; the Claude Agent SDK surfaced this as exit 143.

Reproducible at the time via `cat /sys/fs/cgroup/memory.max` returning `2066325504`.

## Current state

`cat /sys/fs/cgroup/memory.max` now returns `max` (unlimited) and the host shows 19 GB total / 12 GB available. The root cause is no longer reproducible, which is great, but the bump was either:

- (a) a deliberate `docker compose` change since this morning that wasn't documented anywhere I can find, or
- (b) a host-level OrbStack/Docker Desktop setting flip.

I cannot find which.

## Proposed follow-up

- Document the resolved bump in `nerve/bootstrap.py::_build_docker_compose`. If we don't pin a sane minimum memory limit explicitly, the next time someone runs `nerve init` on a fresh host they'll get whatever the daemon's default is.
- Optional: export `NODE_OPTIONS=--max-old-space-size=1500` in the validate-after-change skill for HyperDX repos as a belt-and-suspenders mitigation (the limit only triggers when actually running Node tooling, doesn't hurt otherwise).

## Audit-log evidence

- `~/.nerve/mechanical-actions/audit.jsonl` line 14: 21:55:29Z watcher quiet-tick note about the state-file reset (which itself was a side effect of the broader incident).
- Sessions API `messages` for `impl-af6f01dd`: last assistant turn at 2026-05-28T21:37:45Z is literally "Agent error: Command failed with exit code 143"; same for `impl-2d3f4375` at 21:39:12Z.

Filing this so the next cgroup change has context and so the diagnosis is searchable in memU later.