Split per-app dataset into OS/data layers, add skiff rebuild

Each app now gets two ZFS datasets - a disposable OS layer
(zroot/skiff/appbase/<app>) cloned from base, and a persistent data
layer (zroot/skiff/appdata/<app>) holding releases/current/env/
meta.json/data.db. `skiff rebuild --app|--all` discards and reclones
the OS layer to pick up an OS/toolchain/config-template upgrade,
guarded by the same snapshot-and-rollback safety net as deploy and
config changes; the data layer is never touched.

Also fixes stale docs (builds already run via jexec inside the jail,
not on the host; --iface already auto-aliases the jail's IP, so the
manual ifconfig step and its troubleshooting entry are gone) found
while updating the same sections for this feature.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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