ac-os: add `pull` command + fix oven skipping binary rebuild
- `ac-os pull`: downloads latest OTA kernel from CDN, verifies SHA256,
injects user creds, and flashes to USB — no local build needed (~30s)
- Remove --skip-binary from oven builds: AC_BUILD_NAME and AC_GIT_HASH
are compiled into the binary and change every commit. Skipping the
binary caused devices to show stale build names. The Makefile's CFLAGS
signature check handles incremental rebuilds efficiently already.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>