Start no account's manager to look at its units (hq issue 764)

Every 15 s the failed-units look read the own manager of every account
the declaration creates, and reading one is a login that starts it: six
module accounts' managers on the control node started and stopped all
day, most of its journal. A look only reads (ADR 0240 rule 6) and never
asks an account's manager (ADR 0177).

An account's manager is read now only when the declaration places a
user-scoped unit in it, and only after the machine's own manager says
user@<uid>.service is active, as the apply asks before it. A manager not
running is said away in the units statement, with no failed units; in
liveness its units read unknown with the apply's words, never down. The
operator's account on a desktop is still read while logged in, since a
module places a user unit there (issue 315's Razer daemon stays said).

internal/liveness/runtime.go

@@ -8,6 +8,8 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+
+	"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
@@ -16,9 +18,13 @@ type Runner func(ctx context.Context, name string, args ...string) (string, erro
 
 // Exec reads the container runtime and the service manager through their command lines: one inspect of
 // every container, one show of every unit per manager. **Reads only** — the whole of what this file may
-// ask either of them is `container inspect`, `version` and `show` (ADR 0240 rule 6, and a test holds it).
+// ask either of them is `container inspect`, `version` and `show`, and of the machine's manager whether an
+// account's own is active (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 the machine's manager is asked whether an
+	// account's own runs (system.ManagerRunning).
+	System string
 
 	mu sync.Mutex
 	// cri is the runtime that answered, once one has.
@@ -146,6 +152,19 @@ func (e *Exec) Units(ctx context.Context, scope, user string, units []string) (m
 	}
 	args := []string{"show", "--property=" + unitProperties, "--"}
 	if scope == "user" && user != "" {
+		// **A look starts no account's manager** (novox/hq issue 764, ADR 0177): reading it is a login,
+		// which starts it when none runs. The machine's manager is asked first; a manager not running is
+		// away — its units unread, said unknown with why, never down — as the apply says them waiting.
+		running, exists, err := system.ManagerRunning(ctx, e.System, system.Runner(e.Run), user)
+		switch {
+		case err != nil:
+			return nil, fmt.Errorf("whether %s's own service manager runs could not be read: %w", user, err)
+		case !exists:
+			return nil, fmt.Errorf("the machine has no account %q, whose own manager these units are in", user)
+		case !running:
+			return nil, fmt.Errorf("%s's own service manager is not running: the account is not logged in "+
+				"and does not linger", user)
+		}
 		args = append([]string{"--user", "--machine=" + user + "@"}, args...)
 	}
 	said, err := e.Run(ctx, "systemctl", append(args, units...)...)