Add bin/setup to prepare a repository host

The sync guard could not read the repository database because bsdtar was not
installed on the host, and the first fix was to parse around its absence. The
better answer is for the host to have what the tooling needs: libarchive ships
the library pacman links against without necessarily installing the binary, so
bsdtar being present was an assumption, not a fact.

bin/setup installs the dependencies, enables Docker, creates the state
directory, and installs and enables the release timers -- the steps the README
previously listed by hand. It is idempotent and takes --check to report without
changing anything. Signing credentials and the rclone remote hold secrets, so
it reports on those rather than creating them.

sync-repo goes back to reading the database with bsdtar alone, and says to run
bin/setup when it is missing.

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

README.md

@@ -560,21 +560,21 @@ State files are stored in `/root/.state/`:
 ### Installation
 
 ```bash
-# Copy systemd units
-cp /root/omarchy-pkgs/systemd/*.service /root/omarchy-pkgs/systemd/*.timer /etc/systemd/system/
-
-# Reload systemd
-systemctl daemon-reload
-
-# Enable and start timers
-systemctl enable --now omarchy-check-versions.timer
-systemctl enable --now omarchy-auto-release-edge.timer
-systemctl enable --now omarchy-auto-release-stable.timer
-
-# Create state directory
-mkdir -p /root/.state
+ssh root@<host> 'cd /root/omarchy-pkgs && bin/setup'
 ```
 
+`bin/setup` installs the dependencies, enables Docker, creates the state
+directory, and installs and enables the release timers. It is idempotent, so
+run it again whenever a dependency is added.
+
+```bash
+bin/repo setup --check         # Report what is missing, change nothing
+bin/repo setup --skip-timers   # Prepare the host without the release timers
+```
+
+Signing credentials (`/root/.omarchy/build-credentials`) and the rclone remote
+hold secrets, so setup reports on them rather than creating them.
+
 ### Management
 
 ```bash