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>

pendingattach/pendingattach_test.go

@@ -0,0 +1,35 @@
+package pendingattach
+
+import "testing"
+
+func TestDedupe_CollapsesByFileID(t *testing.T) {
+	rows := []Attachment{
+		{ID: "1", FileID: "a", Ord: 0},
+		{ID: "2", FileID: "b", Ord: 1},
+		{ID: "3", FileID: "a", Ord: 2}, // dup of file a
+	}
+	out := Dedupe(rows)
+	if len(out) != 2 {
+		t.Fatalf("want 2 rows, got %d: %+v", len(out), out)
+	}
+	if out[0].ID != "1" || out[1].ID != "2" {
+		t.Fatalf("first-wins order not preserved: %+v", out)
+	}
+}
+
+func TestDedupe_EmptyFileIDNeverCollapsed(t *testing.T) {
+	rows := []Attachment{
+		{ID: "1", FileID: ""},
+		{ID: "2", FileID: ""},
+	}
+	if got := Dedupe(rows); len(got) != 2 {
+		t.Fatalf("empty FileID rows must not collapse, got %d", len(got))
+	}
+}
+
+func TestDedupe_ShortInputPassthrough(t *testing.T) {
+	rows := []Attachment{{ID: "1", FileID: "a"}}
+	if got := Dedupe(rows); len(got) != 1 {
+		t.Fatalf("single row should pass through, got %d", len(got))
+	}
+}