P2: run.Executor — executus is runnable

The capstone of the run kernel: run.Executor.Run(ctx, RunnableAgent, inv)
ties model resolution + the tool registry + majordomo's agent loop +
context compaction + run-bounding + step/audit instrumentation into one
path, with every host concern behind the nil-safe run.Ports.

- run/executor.go: New(Config{Registry, Models, Defaults, Ports, Compactor,
  ContextTokens, SystemHeader}) + Run -> Result{RunID, Output, Steps, Usage,
  Err}. Budget gate (pre-run), model resolve, Audit StartRun/recorder
  (satisfies RunTally, stamped on inv.RunState), toolbox build, step observer
  (zips tool calls/results -> emitter + recorder.OnStep/OnTool), V10
  detached-MaxRuntime context with caller-cancel merged back, compaction wired
  from ContextTokens×ratio, audit Close + Budget Commit on a detached cleanup
  ctx. Zero Ports = a bounded in-memory run (gadfly's case).
- run/executor_test.go: hermetic end-to-end run against majordomo's fake
  provider (hello-world), Budget-rejection (no model call), Audit-port wiring
  (StartRun + Close with terminal status/output). All green under -race.
- examples/minimal upgraded to the real "hello, agentic world" (~15 lines:
  Configure tiers -> run.New -> Run -> print). README/CLAUDE.md updated.

Remaining P2 follow-ups (incremental): wire Critic/Checkpointer/PaletteSource/
Delivery into the loop, multi-phase Pipelines, and the no-tools direct path.

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

examples/minimal/main.go

@@ -1,27 +1,49 @@
-// 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).
+// Command minimal is executus's "hello, agentic world": wire a model resolver,
+// a tool registry, and the run executor, then run an agent. With no batteries
+// (Audit/Budget/Critic/Checkpointer/Palette/Delivery all nil) this is a
+// bounded, in-memory run — the light-host shape (gadfly's case).
+//
+// Run it with a provider key for the configured tier, e.g.
+//
+//	ANTHROPIC_API_KEY=sk-... go run ./examples/minimal
+//
+// Override a tier from the environment without touching code, e.g.
+//
+//	EXECUTUS_MODEL_TIER_FAST=openai/gpt-4o-mini ANTHROPIC_API_KEY= OPENAI_KEY=sk-... go run ./examples/minimal
 package main
 
 import (
 	"context"
 	"fmt"
+	"log"
 
 	"gitea.stevedudenhoeffer.com/steve/executus/config"
-	"gitea.stevedudenhoeffer.com/steve/executus/fanout"
+	"gitea.stevedudenhoeffer.com/steve/executus/model"
+	"gitea.stevedudenhoeffer.com/steve/executus/run"
+	"gitea.stevedudenhoeffer.com/steve/executus/tool"
 )
 
 func main() {
-	cfg := config.Env("EXECUTUS_") // e.g. EXECUTUS_FANOUT_MAX_CONCURRENT=8
-	max := cfg.Int("fanout.max_concurrent", 4)
+	// 1. Configure model tiers: live values come from the environment
+	//    (EXECUTUS_MODEL_TIER_<NAME>), falling back to these defaults.
+	model.Configure(config.Env("EXECUTUS_"), map[string]string{
+		"fast":     "anthropic/claude-haiku-4-5",
+		"thinking": "anthropic/claude-opus-4-8",
+	}, 0)
 
-	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 })
+	// 2. Build the executor: a tool registry + the model resolver. No batteries.
+	ex := run.New(run.Config{
+		Registry: tool.NewRegistry(),
+		Models:   model.ParseModelForContext,
+	})
 
-	for _, r := range results {
-		fmt.Printf("%-6s -> %d (err=%v)\n", items[r.Index], r.Value, r.Err)
+	// 3. Run an agent and print its answer.
+	res := ex.Run(context.Background(),
+		run.RunnableAgent{Name: "assistant", SystemPrompt: "You are concise.", ModelTier: "fast"},
+		tool.Invocation{RunID: "demo-1", CallerID: "local"},
+		"In one sentence, what is an agent harness?")
+	if res.Err != nil {
+		log.Fatalf("run failed: %v", res.Err)
 	}
+	fmt.Println(res.Output)
 }