Land feature 57's node-engine work on its release branch

The node-engine renders the feature's templates, collections and moved
providers' files, on the SDK the feature's modules build on (feature 57,
config and provisioning).

internal/units/exec.go

@@ -5,6 +5,8 @@ import (
 	"errors"
 	"fmt"
 	"strings"
+
+	"github.com/novox/mesh-host/internal/system"
 )
 
 // Runner runs a command and answers what it printed — the apply's own (apply.ExecRunner), so a look reads
@@ -12,8 +14,9 @@ import (
 type Runner func(ctx context.Context, name string, args ...string) (string, error)
 
 // Exec reads the service managers through systemctl and the package database through the system's own
-// owner query. **Reads only**: `list-units`, `show` and the owner query are the whole of what it asks
-// (ADR 0240 rule 6, and a test holds it).
+// owner query. **Reads only**: `list-units`, `show`, the owner query, and — of the machine's manager and
+// user database — whether an account's manager is active, are the whole of what it asks (ADR 0240 rule 6,
+// and a test holds it).
 type Exec struct {
 	Run Runner
 	// System is what the host was built for; it says how a file's package is asked. A system with no
@@ -103,3 +106,10 @@ func (e Exec) PackageOwning(ctx context.Context, path string) (string, bool, err
 	}
 	return "", false, nil
 }
+
+// UserManagerRunning asks the machine's manager whether an account's own runs, as the apply does before it
+// touches a user-scoped unit (system.UserManagerRunning; novox/hq ADR 0177, issue 764). A system with no
+// manager per account cannot say, and is an error: that manager is said unread.
+func (e Exec) UserManagerRunning(ctx context.Context, account string) (running, exists bool, err error) {
+	return system.ManagerRunning(ctx, e.System, system.Runner(e.Run), account)
+}