:bug: AgentWorkflowRun status patch races with child AgentRun phase initialization

## Summary

`AgentWorkflowRunReconciler` can observe a newly created child `AgentRun` before the `AgentRun` controller initializes `status.phase`. The workflow-run controller mirrors that empty phase into `AgentWorkflowRun.status.stages[].phase`, and the API server rejects the status update because the CRD phase field only permits `Pending`, `Running`, `Succeeded`, or `Failed`.

## Reproduction

```sh
KUBEBUILDER_ASSETS="$(pwd)/$(./bin/setup-envtest use 1.36 -p path --bin-dir ./bin)" \
GOCACHE=/tmp/agentic-controller-gocache \
go test ./internal/controller
```

## Observed error

```
status.stages[0].phase: Unsupported value: "":
supported values: "Pending", "Running", "Succeeded", "Failed"
```

The failure is transient and occurs during reconciliation after the child `AgentRun` is created. It can cause the workflow-run status patch to fail and delay or disrupt status propagation.

## Root cause

The workflow-run reconciler unconditionally copied:

``
go
stageStatus.Phase = agentRun.Status.Phase
```

When the child `AgentRun` had not yet initialized its phase, this wrote an invalid empty value to the parent status.

## Proposed fix

Only mirror the child phase when it is non-empty, preserving the stage's existing `Pending` value during the initialization window:

``
go
if agentRun.Status.Phase != "" {
stageStatus.Phase = agentRun.Status.Phase
}
```

Add or retain an envtest regression covering the newly-created-child timing window. A local candidate guard and snapshot-related regression test are currently being developed alongside this issue.

Start in the AgentWorkflowRunReconciler under internal/controller and run the provided envtest command to reproduce the newly-created-child timing window. Inspect the existing status propagation coverage and add or update the regression so status updates remain valid while the child phase is uninitialized; done means the test passes without an unsupported empty phase.

Written by the indexing model from the issue text.