COPY multi-source support, configurable rootfs size, build fixes

- COPY now supports multiple sources: COPY a.txt b.txt /dest/
  Last argument is always destination (matches Dockerfile semantics).
- COPY resolves relative destinations against current WORKDIR.
- WRENN_DEFAULT_ROOTFS_SIZE env var (e.g. 5G, 2Gi, 1000M, 512Mi)
  controls template rootfs expansion. Used both at agent startup
  (EnsureImageSizes) and after FlattenRootfs (shrink then re-expand).
- Pre-build now sets WORKDIR /home/wrenn-user after USER switch.
- Extracted archive files get chmod a+rX for readability.
- Path traversal validation on COPY sources.

.env.example

@@ -12,6 +12,7 @@ WRENN_HOST_LISTEN_ADDR=:50051
 WRENN_DIR=/var/lib/wrenn
 WRENN_HOST_INTERFACE=eth0
 WRENN_CP_URL=http://localhost:8080
+WRENN_DEFAULT_ROOTFS_SIZE=5Gi
 
 # Lago (billing — external service)
 LAGO_API_URL=http://localhost:3000