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>

examples/minimal/main.go

@@ -0,0 +1,27 @@
+// Command minimal demonstrates executus's standalone core primitives available
+// today (P0): the config seam + bounded fan-out. The full zero-config "agentic
+// in ~12 lines" example arrives once the model, tool, and run packages land
+// (P1–P3).
+package main
+
+import (
+	"context"
+	"fmt"
+
+	"gitea.stevedudenhoeffer.com/steve/executus/config"
+	"gitea.stevedudenhoeffer.com/steve/executus/fanout"
+)
+
+func main() {
+	cfg := config.Env("EXECUTUS_") // e.g. EXECUTUS_FANOUT_MAX_CONCURRENT=8
+	max := cfg.Int("fanout.max_concurrent", 4)
+
+	items := []string{"alpha", "beta", "gamma", "delta"}
+	results := fanout.Run(context.Background(), items,
+		fanout.Options[string]{MaxConcurrent: max},
+		func(_ context.Context, s string) (int, error) { return len(s), nil })
+
+	for _, r := range results {
+		fmt.Printf("%-6s -> %d (err=%v)\n", items[r.Index], r.Value, r.Err)
+	}
+}