fix(instance): never serve a container-id hostname as the instance name

os.Hostname() inside the Maestro container is the bare container id
(e.g. 6079761d6e4d) — a hex-only hostname now falls back to 'maestro'
so the public profile stays meaningful out of the box. DCNTRLIZED_INSTANCE_NAME
and instance.json still win.

src/lib/instance/instance.go

@@ -83,12 +83,31 @@ func Save(dataRoot string, id Identity) error {
 	return nil
 }
 
+// osHostname is swapped in tests.
+var osHostname = os.Hostname
+
 func defaultIdentity() Identity {
-	name := os.Getenv("DCNTRLIZED_INSTANCE_NAME")
+	if name := os.Getenv("DCNTRLIZED_INSTANCE_NAME"); name != "" {
-	if name == "" {
-		if host, err := os.Hostname(); err == nil {
-			name = host
-		}
-	}
 		return Identity{Name: name}
+	}
+	// Maestro normally runs containerized, where os.Hostname() is the bare
+	// container id — useless (and mildly leaking) as a public display name.
+	if host, err := osHostname(); err == nil && !looksLikeContainerID(host) {
+		return Identity{Name: host}
+	}
+	return Identity{Name: "maestro"}
+}
+
+// looksLikeContainerID reports whether s is a hex-only Docker-style container
+// id or short id.
+func looksLikeContainerID(s string) bool {
+	if len(s) < 6 {
+		return false
+	}
+	for _, r := range s {
+		if !(r >= '0' && r <= '9' || r >= 'a' && r <= 'f' || r >= 'A' && r <= 'F') {
+			return false
+		}
+	}
+	return true
 }

src/lib/instance/instance_test.go

@@ -0,0 +1,91 @@
+// Copyright (C) 2026 Dcntrlized
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published
+// by the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package instance
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadPrefersFileOverEnv(t *testing.T) {
+	dir := t.TempDir()
+	if err := Save(dir, Identity{Name: "home-node", Title: "Home"}); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("DCNTRLIZED_INSTANCE_NAME", "from-env")
+
+	id := Load(dir)
+	if id.Name != "home-node" || id.Title != "Home" {
+		t.Fatalf("got %+v", id)
+	}
+}
+
+func TestDefaultIdentityFallbacks(t *testing.T) {
+	orig := osHostname
+	defer func() { osHostname = orig }()
+
+	t.Run("env wins", func(t *testing.T) {
+		t.Setenv("DCNTRLIZED_INSTANCE_NAME", "edge-1")
+		if id := defaultIdentity(); id.Name != "edge-1" {
+			t.Fatalf("got %q", id.Name)
+		}
+	})
+
+	t.Run("hostname used when human-readable", func(t *testing.T) {
+		os.Unsetenv("DCNTRLIZED_INSTANCE_NAME")
+		osHostname = func() (string, error) { return "archlinux", nil }
+		if id := defaultIdentity(); id.Name != "archlinux" {
+			t.Fatalf("got %q", id.Name)
+		}
+	})
+
+	t.Run("container-id hostname replaced", func(t *testing.T) {
+		os.Unsetenv("DCNTRLIZED_INSTANCE_NAME")
+		for _, host := range []string{"6079761d6e4d", "6079761d6e4dabcdef1234567890abcdef1234567890abcdef1234567890abcd"} {
+			osHostname = func() (string, error) { return host, nil }
+			if id := defaultIdentity(); id.Name != "maestro" {
+				t.Fatalf("host %q: got %q", host, id.Name)
+			}
+		}
+	})
+}
+
+func TestLooksLikeContainerID(t *testing.T) {
+	cases := map[string]bool{
+		"6079761d6e4d":                        true,
+		"ABCDEF0123456789abcdef0123456789":    true,
+		"archlinux":                           false,
+		"maestro-1":                           false,
+		"abc":                                 false,
+		"":                                    false,
+	}
+	for s, want := range cases {
+		if got := looksLikeContainerID(s); got != want {
+			t.Errorf("looksLikeContainerID(%q) = %v, want %v", s, got, want)
+		}
+	}
+}
+
+func TestSaveRejectsEmptyName(t *testing.T) {
+	dir := t.TempDir()
+	if err := Save(dir, Identity{}); err == nil {
+		t.Fatal("expected error for empty name")
+	}
+	if _, err := os.Stat(filepath.Join(dir, "instance.json")); !os.IsNotExist(err) {
+		t.Fatal("no file should be written for an invalid identity")
+	}
+}