feat: cloud adapters E2B/Modal and billing export (SAND-WP-0010)

Add credentialed E2B and Modal extensions, burst routing fallback,
fin-hub meter export hook, BYOK docs, and 77 tests.

scripts/smoke-cloud-adapter.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+# Operator smoke for cloud adapters — requires provider credentials.
+set -euo pipefail
+
+PROVIDER="${1:-e2b}"
+case "$PROVIDER" in
+  e2b)
+    PROFILE="profile.e2b-burst"
+    if [[ -z "${E2B_API_KEY:-}" ]]; then
+      echo "E2B_API_KEY not set — skipping live smoke" >&2
+      exit 0
+    fi
+    ;;
+  modal)
+    PROFILE="profile.modal-gpu"
+    if [[ -z "${MODAL_TOKEN_ID:-}" ]]; then
+      echo "MODAL_TOKEN_ID not set — skipping live smoke" >&2
+      exit 0
+    fi
+    ;;
+  *)
+    echo "Usage: $0 [e2b|modal]" >&2
+    exit 1
+    ;;
+esac
+
+echo "Smoke: sandboxer create --profile $PROFILE"
+STATUS=$(sandboxer create --profile "$PROFILE" --project sand-boxer)
+ID=$(echo "$STATUS" | python3 -c "import sys,json; print(json.load(sys.stdin)['sandbox_id'])")
+echo "Created: $ID"
+sandboxer destroy "$ID"
+echo "Destroyed: $ID"