Make /opt/moonarch root-owned for multi-user support

Remove chown from archinstall custom-commands so the repo stays
root:root. Use sudo for git operations in update.sh. Any user with
sudo can now run moonarch-update without owning the repo.

DECISIONS.md

@@ -0,0 +1,13 @@
+# Decisions
+
+## 2026-03-29 – /opt/moonarch stays root-owned, no chown to user
+- **Who**: Dominik, Ragnar
+- **Why**: Multi-user system — chown to UID 1000 locks out other users from moonarch-update
+- **Tradeoffs**: sudo required for git operations in update.sh vs. simpler user-owned repo
+- **How**: Repo stays at /opt/moonarch owned by root:root. update.sh uses `sudo git` for fetch/pull. All scripts already use sudo for system-level operations, so this is consistent.
+
+## 2026-03-29 – Add transform.sh for existing Arch+Wayland systems
+- **Who**: Dominik, Ragnar
+- **Why**: Users with existing Arch+Wayland setups should be able to adopt Moonarch without reinstalling
+- **Tradeoffs**: Hard overwrite of all configs (user + system) vs. selective/merge approach — chose hard overwrite for simplicity and consistency
+- **How**: New transform.sh with pre-flight summary, backup, DM conflict resolution, and --dry-run flag. Shared helpers extracted to lib.sh.