Refuse build steps under a free-space floor and release workspaces, so the build modules stop filling a build node's disk (issue 1839)

mesh-build-go, mesh-build-node and mesh-build-image refuse every fetch, test
and build, and say unhealthy, while their work or cache filesystem has less
than 20 GiB free (build-agent's floor), so the builder chooses another node
instead of filling this one. Each serves build_<x>_release, removing a
workspace once its caller's last step answered; a failed fetch removes its
own; a workspace never released goes two hours after its last use (ADR 0509)
in place of a day, never while a step holds it. Pins mesh-sdk's
fix/1839-build-floor (buildstep.Floor, buildstep.Workspaces).

modules/mesh-build-node/cmd/mesh-build-node/module.go

@@ -59,8 +59,14 @@ type Module struct {
 	// unhealthy, while it is missing or older than BubblewrapLeast. Nil probes nothing.
 	Bubblewrap func(ctx context.Context) (string, error)
 
-	mu      sync.Mutex // guards Recipe
-	install sync.Mutex // one install at a time
+	// Floor refuses every step while the work or toolchain cache filesystem has less free than it (issue 1839); main
+	// sets floorOf(node), and a zero Floor checks nothing.
+	Floor buildstep.Floor
+
+	spacesOnce sync.Once
+	workspaces *buildstep.Workspaces
+	mu         sync.Mutex // guards Recipe
+	install    sync.Mutex // one install at a time
 }
 
 func (m *Module) recipe() Recipe {
@@ -122,6 +128,7 @@ func (m *Module) Start(ctx context.Context) {
 		if err := m.Prepare(ctx); err != nil && m.Say != nil {
 			m.Say("the toolchain: %v", err)
 		}
+		m.CleanWorkspaces(m.now())
 		select {
 		case <-ctx.Done():
 			return
@@ -138,6 +145,9 @@ func (m *Module) Health(ctx context.Context) health.Answer {
 	if err != nil {
 		return health.Unhealthy(err.Error())
 	}
+	if err := m.room(); err != nil {
+		return health.Unhealthy(err.Error())
+	}
 	return health.Healthy(fmt.Sprintf("node %s installed, the version the module builds with (recipe %s)", installed, m.recipe().Version()))
 }
 
@@ -150,6 +160,10 @@ func (m *Module) Long(tool string, run func(ctx context.Context, args map[string
 			if err := m.Prepare(ctx); err != nil {
 				return nil, err
 			}
+			if err := m.room(); err != nil {
+				return nil, err
+			}
+			defer m.holdArg(args)()
 			return run(ctx, args)
 		})
 		if err != nil {
@@ -166,6 +180,10 @@ func (m *Module) Short(run func(ctx context.Context, args map[string]any) (any,
 		if err := m.Prepare(m.Ctx); err != nil {
 			return nil, err
 		}
+		if err := m.room(); err != nil {
+			return nil, err
+		}
+		defer m.holdArg(args)()
 		return run(m.Ctx, args)
 	}
 }