User templates can now be added without rebuilding: drop a script into
~/.config/motd-assist/templates/ and reference it from a preset via
"install": "<filename>". Same-name files override built-ins; new files
are picked up by the running TUI via 'r'. `motd-assist templates` lists
known templates and creates the drop-in directory.
Apply plans now embed the resolved template content (Plan.ResolveContents)
so the sudo/root process never needs to read user config; snapshots store
installed script content so restore works under sudo as well. Validation
rejects plans with unresolved content.
