run: PostRun detached ctx + panic-isolated Cleanup (gadfly #12)

Two convergent gadfly refinements on the PostRun wiring:
- PostRun now runs on detach(ctx), not the caller's ctx — a finished/cancelled
  caller no longer aborts artifact production (3-model: glm-5.2/minimax/deepseek).
- Cleanup is panic-isolated via safeCleanup (recover+log), matching runPostRun, so
  a misbehaving teardown can't clobber an otherwise-successful run (deepseek).

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

run/session.go

@@ -74,3 +74,15 @@ func (s *runSession) AttachImages(text string, images ...llm.ImagePart) {
 	}
 	s.mailbox.push(llm.UserParts(parts...))
 }
+
+// safeCleanup runs a SessionTools.Cleanup with panic isolation, so a misbehaving
+// teardown (temp-dir removal, handle close) can't clobber an otherwise-successful
+// run via the executor's top-level recover.
+func safeCleanup(fn func()) {
+	defer func() {
+		if r := recover(); r != nil {
+			slog.Error("run: session Cleanup panicked", "panic", r)
+		}
+	}()
+	fn()
+}