Fix nine defects in the push/sync path found in review

The worst was fatal: push passed --skip-prod-check to upload-prebuilt, which
forwards every argument to sign, promote and update as well, and sign rejects
unknown options. Every non-dry-run push and deploy would have uploaded and
verified its artifacts and then failed before signing. upload-prebuilt now
routes publishing flags to sync alone.

The partial-tree guard was weaker than it looked:

  - it counted archive files locally against package names in the remote
    database, and this tree keeps two versions per package, so a checkout with
    a spare version of half the repository could pass while still hiding
    hundreds of packages. It now compares package-name sets and lists what
    would be hidden.
  - it treated any unreadable remote as an empty one, so an auth failure or a
    corrupt database disabled it. Only rclone's "directory not found" now
    counts as a fresh mirror; every other failure aborts.

Also:

  - sync had no set -e, so a failed package upload fell through to publishing
    the database, advertising packages that were never uploaded. Each transfer
    is now checked before the next step.
  - --package with no names silently meant "every package", which under --yes
    could publish everything from one unset variable in a script.
  - push now refuses to run when the host has packages staged from an earlier
    failure, since publishing would sign and promote those too.
  - epoch versions contain a colon, which rsync reads as host:path, so no
    package with an epoch could be transferred. Sources are ./-prefixed.
  - remote paths are quoted for the remote shell.
  - sync spun forever on a missing option value.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -218,6 +218,11 @@ Split packages are selected by their own names, not their pkgbase — pushing
 `nvidia-580xx-utils` does not carry `nvidia-580xx-dkms` along. Omit `--package` to
 push everything built.
 
+Publishing signs and promotes everything staged on the host, not just what this
+push uploaded, so `push` stops when it finds packages already staged there —
+usually leftovers from a failed run. Remove them on the host, or pass
+`--include-staged` to publish them too.
+
 ### Sync AUR PKGBUILDs
 
 ```bash