run: wire SessionToolFactory + PostRun artifacts + AttachImages

The session-tool TYPES already lived in tool/ (P4 move) but the executor never
used them. This wires them, unblocking artifact-producing host surfaces (mort's
chat API / chatbot / .skill / scaddy) to run on executus:

- run/session.go: steerMailbox (thread-safe message queue) + runSession
  (tool.AgentSession over it: AttachImages → a user-role multimodal message
  injected before the agent's next step) + runPostRun (panic-isolated hook call).
- executor: create the mailbox + set inv.AttachImages BEFORE the toolbox build;
  add inv.ExtraTools + a SessionToolFactory's per-run Tools to the toolbox; defer
  its Cleanup; merge the session mailbox with the critic's nudges into ONE
  WithSteer; after the run, call PostRun with the full transcript
  (runRes.Messages) → Result.PostRunResult (best-effort, never fails the run).
- run.Result += PostRunResult *tool.PostRunResult.
- dropped the now-dead criticBinding.steerOptions (superseded by drainSteer).

Tests: a factory whose PostRun emits an artifact from the output+transcript +
Cleanup lands on Result.PostRunResult; a factory-added tool is callable.

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

run/critic.go

@@ -114,11 +114,11 @@ func (b *criticBinding) maxStepsOption(base int) agent.Option {
 	})
 }
 
-// steerOptions returns the agent RunOptions that drain the critic's steer
-// messages into the loop. Empty when there is no critic.
-func (b *criticBinding) steerOptions() []agent.RunOption {
+// drainSteer returns the critic's queued steer messages (nil-safe), so the
+// executor can merge them with the session steer mailbox into one WithSteer.
+func (b *criticBinding) drainSteer() []llm.Message {
 	if b == nil {
 		return nil
 	}
-	return []agent.RunOption{agent.WithSteer(b.h.Steer)}
+	return b.h.Steer()
 }