P0: stand up executus harness module above majordomo

Batteries-included agent-harness base, extracted from mort's agent layer.
This first cut establishes the module + the zero-coupling core primitives:

- lane, dispatchguard, pendingattach, run/progress.go: moved verbatim from mort
- config: host config Source seam + env-var default (nil-safe helpers)
- deliver: output-egress seam + Discard/Stdout defaults
- identity: AdminPolicy + MemberResolver seams (nil-safe)
- fanout: programmatic N×M swarm (bounded global + per-key concurrency)
- README/CLAUDE.md with the vibe-coded banner; CI with Go gates +
  the "core stays majordomo+stdlib only" invariant

Core builds with stdlib only today; majordomo enters at P1 (model/structured).
go build/vet/test -race all green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

config/config_test.go

@@ -0,0 +1,56 @@
+package config
+
+import "testing"
+
+func TestEnvNameMapping(t *testing.T) {
+	e := envSource{prefix: "GADFLY_"}
+	cases := map[string]string{
+		"models":                        "GADFLY_MODELS",
+		"model.tier.fast.max_steps":     "GADFLY_MODEL_TIER_FAST_MAX_STEPS",
+		"provider-concurrency":          "GADFLY_PROVIDER_CONCURRENCY",
+		"a/b.c":                         "GADFLY_A_B_C",
+	}
+	for key, want := range cases {
+		if got := e.envName(key); got != want {
+			t.Errorf("envName(%q) = %q, want %q", key, got, want)
+		}
+	}
+}
+
+func TestEnvReadsAndDefaults(t *testing.T) {
+	t.Setenv("EX_MODELS", "a,b,c")
+	t.Setenv("EX_MAX", "12")
+	t.Setenv("EX_RATIO", "0.7")
+	t.Setenv("EX_ON", "true")
+	t.Setenv("EX_BLANK", "")
+	s := Env("EX_")
+
+	if got := s.String("models", "def"); got != "a,b,c" {
+		t.Errorf("String = %q", got)
+	}
+	if got := s.String("blank", "def"); got != "def" {
+		t.Errorf("blank String should fall back to default, got %q", got)
+	}
+	if got := s.String("missing", "def"); got != "def" {
+		t.Errorf("missing String = %q", got)
+	}
+	if got := s.Int("max", 1); got != 12 {
+		t.Errorf("Int = %d", got)
+	}
+	if got := s.Int("models", 1); got != 1 {
+		t.Errorf("unparseable Int should default, got %d", got)
+	}
+	if got := s.Float("ratio", 1); got != 0.7 {
+		t.Errorf("Float = %v", got)
+	}
+	if got := s.Bool("on", false); got != true {
+		t.Errorf("Bool = %v", got)
+	}
+}
+
+func TestNilSafeHelpers(t *testing.T) {
+	var s Source // nil
+	if String(s, "k", "d") != "d" || Int(s, "k", 7) != 7 || Float(s, "k", 1.5) != 1.5 || !Bool(s, "k", true) {
+		t.Fatal("nil Source helpers must return defaults")
+	}
+}