Feat v0.9.7 workflow starlark write ops

Add workflow.start(), workflow.advance(), workflow.cancel(), and
workflow.submit_signoff() to the Starlark workflow module. Extract
WorkflowEngine interface in sandbox package to break the circular
import between workflow and sandbox packages.

- WorkflowEngine interface (Start/Advance/Cancel/SubmitSignoff)
- instanceToDict/signoffToDict/dictToJSON shared helpers
- Runner.SetWorkflowEngine setter + main.go wiring
- 6 new unit tests with mock engine

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/sandbox/runner.go

@@ -82,6 +82,7 @@ type Runner struct {
 	workspaceRoot   string               // empty = workspace module unavailable
 	workspaceQuota  int                  // MB, 0 = unlimited
 	capabilities    map[string]bool      // detected environment capabilities
+	wfEngine        WorkflowEngine       // nil = workflow write ops unavailable
 }
 
 // NewRunner creates a runner with the given sandbox and dependencies.
@@ -139,6 +140,12 @@ func (r *Runner) SetCapabilities(caps map[string]bool) {
 	r.capabilities = caps
 }
 
+// SetWorkflowEngine attaches the workflow engine for write operations
+// (start/advance/cancel/submit_signoff) in the workflow Starlark module.
+func (r *Runner) SetWorkflowEngine(e WorkflowEngine) {
+	r.wfEngine = e
+}
+
 // SetAllowPrivateIPs disables the SSRF check that blocks connections to
 // private/loopback IPs. For self-hosted environments where extensions
 // reach internal services. Controlled by EXT_ALLOW_PRIVATE_IPS env var.
@@ -396,7 +403,7 @@ func (r *Runner) buildModulesWithLibCtx(ctx context.Context, packageID string, m
 			filesLevel = 2
 
 		case models.ExtPermWorkflowAccess:
-			modules["workflow"] = BuildWorkflowModule(ctx, r.stores)
+			modules["workflow"] = BuildWorkflowModule(ctx, r.stores, r.wfEngine, rc)
 
 		case models.ExtPermConnectionsRead:
 			if r.connResolver != nil && rc != nil && rc.UserID != "" {