feat: drop-in support for custom templates

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.

internal/motd/state.go

@@ -31,7 +31,8 @@ type (
 	}
 	InstallOp struct {
 		Name    string `json:"name"`
-		Source  string `json:"source"`
+		Source  string `json:"source,omitempty"`
+		Content string `json:"content,omitempty"`
 		Enabled bool   `json:"enabled"`
 	}
 	RemoveOp struct {