refactor(rootfs): split the tarball packer into pack.rs

rootfs.rs mixed the cache lifecycle with the tree-to-tarball
writer that persisting relies on. The packer moves to pack.rs
beside extract.rs: pack_rootfs becomes crate-internal, its
partial-cleanup test moves with it. No behavior change.

AGENTS.md

@@ -35,6 +35,7 @@ Rules:
   - `guest` - the guest init script (`crates/ecrou/src/guest.rs`)
   - `rootfs` - cache-aware preparation and persistence
     (`crates/ecrou/src/rootfs.rs`)
+  - `pack` - rootfs tarball packing (`crates/ecrou/src/pack.rs`)
   - `distro` - distro definitions and mirrors (`crates/ecrou/src/distro.rs`)
   - `download` - image download (`crates/ecrou/src/download.rs`)
   - `extract` - rootfs extraction (`crates/ecrou/src/extract.rs`)