Init scripts: parse netmask + gateway DNS instead of hardcoded /24 + 8.8.8.8

[aniketmaurya]

Context

CodeRabbit flagged two issues on `scripts/ci/preset-init.sh` in #271:

1. Hardcoded `/24` netmask (preset-init.sh:64) — the kernel `ip=` cmdline format includes a netmask field at position 4, but we always assume /24. Breaks routing on non-/24 networks.

2. Hardcoded Google DNS (preset-init.sh:66-68) — overwrites `/etc/resolv.conf` with `8.8.8.8` / `8.8.4.4` regardless of what the network supplies. Fails on locked-down/airgapped networks.

Both findings are valid. Skipped from #271 because the same patterns exist in the existing openclaw `/init` (builder.py::_base_init_script) — fixing only the layered preset init creates asymmetric behavior.

Fix

Apply CodeRabbit's proposed fixes to both init scripts so layered presets and openclaw stay in sync:

• Parse `NETMASK` from `ip=::::...` and convert to CIDR prefix via a `netmask_to_prefix()` shell helper.
• Use `GATEWAY` as the primary DNS resolver; only fall back to public 8.8.8.8/8.8.4.4 when no gateway is parsed.

Acceptance

• VMs on non-/24 subnets (e.g. /16, /20) get correct routing.
• VMs on networks where the gateway is also a DNS resolver use it; only fall through to Google DNS when the gateway is empty.
• Existing /24 + 8.8.8.8 behavior preserved as fallback for current SmolVM-managed networking (which sets a /24 explicitly).

Files

• `scripts/ci/preset-init.sh` — layered presets
• `src/smolvm/images/builder.py::_base_init_script` — openclaw via the heredoc-generated /init

[aniketmaurya]

@codex please work on this

[chatgpt-codex-connector]

Summary

• Added netmask_to_prefix() to the layered preset init script, parse the ip= netmask field, and use the resulting CIDR prefix instead of always adding /24.
• Updated preset DNS setup to use the parsed gateway as the resolver, falling back to 8.8.8.8 / 8.8.4.4 only when the gateway is empty.
• Applied the same netmask parsing and gateway DNS behavior to the builder-generated base init script so OpenClaw and layered presets stay aligned.
• Added regression tests that check both init script paths parse the netmask, use ${PREFIX}, prefer gateway DNS, and no longer hardcode ${GUEST_IP}/24.
• Committed the changes as adac9ae and created a pull request with the requested follow-up fix.

Testing

• ✅ uv run ruff check tests/test_build.py src/smolvm/images/builder.py
• ✅ sh -n scripts/ci/preset-init.sh
• ✅ set -e; uv run python - <<'PY' >/tmp/smolvm-init.sh from smolvm.images.builder import ImageBuilder print(ImageBuilder()._default_init_script()) PY sh -n /tmp/smolvm-init.sh
• ✅ uv run python -m pytest tests/test_build.py -q
• ⚠️ uv run python -m pytest -q — 883 passed and 13 skipped, but 2 existing network tests failed because this environment cannot write /proc/sys/net/ipv4/ip_forward (Read-only file system).

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