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>

CLAUDE.md

@@ -43,12 +43,13 @@ CORE (majordomo + stdlib):
   fanout/   programmatic N×M swarm                       [P0 ✓]
   deliver/  output egress seam (+ Discard/Stdout)        [P0 ✓]
   identity/ caller identity seams                        [P0 ✓]
-  run/      run-loop mechanics + RunnableAgent DTO +     [P2 wip]
-            nil-safe run.Ports (Audit/Budget/Critic/
-            Checkpointer/PaletteSource) + step
-            instrumentation (steps.go) done; the
-            agentexec+skillexec -> run.Executor MERGE
-            (consuming Ports) is the remaining P2 work    [P2]
+  run/      run.Executor is RUNNABLE: model-resolve +    [P2 core ✓]
+            toolbox + majordomo loop + compaction +
+            run-bounding (V10 detached timeout) + step/
+            audit observers + Budget gate; RunnableAgent
+            DTO + nil-safe run.Ports. Follow-ups: wire
+            Critic/Checkpointer/PaletteSource/Delivery,
+            Phases, and the no-tools direct path         [P2]
   dispatchguard/  loop/depth/fan-out caps                [P0 ✓]
   pendingattach/  attachment dedupe                      [P0 ✓]
   tool/     registry + 3-stage permissions + ssrf        [P1 ✓]