test(smolmachines): verify Linux sandbox escape gate + fix platform check

Empirical verification on NixOS/Linux with /dev/kvm + smolvm 1.4.7:

  1. DB path confirmed: ~/.local/share/smolvm/server/smolvm.db exists,
     schema matches (vms table, data BLOB with allowed_cidrs JSON field).

  2. smolvm 1.4.7 still silently drops --allow-cidr with --from
     (allowed_cidrs=None in DB after create). force_allowlist patch
     is still necessary and correct.

  3. All 5 sandbox-escape attacks blocked with BOT_BOTTLE_BACKEND=smolmachines
     on Linux: hostname/IP allowlist, HTTP DLP, DNS exfil, git-gate gitleaks.

Changes:
- test_sandbox_escape: remove darwin-only guard; allow linux too
- contrib/claude/Dockerfile: add dnsutils (dig) for attack 4 DNS test

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bot_bottle/contrib/claude/Dockerfile

@@ -21,7 +21,7 @@ FROM node:22-slim
 # to it) works against egress's bumped TLS without the agent needing
 # local DNS.
 RUN apt-get update \
-  && apt-get install -y --no-install-recommends git ca-certificates curl ripgrep iproute2 \
+  && apt-get install -y --no-install-recommends git ca-certificates curl ripgrep iproute2 dnsutils \
   && rm -rf /var/lib/apt/lists/*
 
 # App-specific deps. Python isn't required by claude-code itself