Structured workflow turn interrupted mid-stream recovers as `skipped` instead of completing with output

## Summary

When a **structured** workflow turn (a `ThinkWorkflow` step driven by `step.prompt({ output })`) is interrupted mid-stream (DO eviction / deploy churn / SIGKILL), chat recovery reconciles it as **`skipped`** rather than re-running it to completion. The workflow then surfaces a `ThinkPromptSkippedError` (`packages/think/src/workflows.ts:67`, `:231`) instead of completing with the validated structured output.

This was discovered while adding e2e recovery coverage. The new e2e (`packages/think/src/e2e-tests/workflow-recovery.test.ts`) deliberately locks in only the guarantee that holds today — the workflow is **unblocked (terminal, not hung)** and the **notification drain replays** the submission's terminal status after restart — and documents this gap inline (see the `NOTE (deferred)` at `workflow-recovery.test.ts:317-324`).

## Expected vs actual

- **Expected:** a structured workflow turn interrupted mid-stream is recovered (continued/retried) and the workflow `step.prompt` resolves with the structured output, same as a non-interrupted run.
- **Actual:** recovery resolves the submission as `skipped`; the workflow step rejects with `ThinkPromptSkippedError` and the workflow reaches a terminal (non-`complete`) state.

## Root cause (initial analysis)

Two interacting behaviors in `packages/think/src/think.ts`:

1. **`continueLastTurn` skips on an interrupted assistant leaf.** When the persisted leaf is a mid-stream (interrupted) assistant message, the chat-recovery continuation treats the turn as already-progressed and **skips** rather than re-running it. For a free-form chat turn that's reasonable (don't double-emit), but for a structured turn it means the turn never re-runs to produce the schema-shaped output.
2. **`_completeRecoveredSubmission` carries no structured output** (`packages/think/src/think.ts:10024`, and its call sites ~`9290/9572/9805/9899/9918/9944/10156/10181`). A recovered submission has no path to reconstruct/re-derive the structured `output`, so even when reconciled it cannot satisfy the `step.prompt({ output })` contract.

## Why it's deferred

A correct fix touches the recovery continuation path fairly broadly (distinguishing structured turns so they re-run to produce output, and threading the structured output through `_completeRecoveredSubmission`), which is higher-risk than the surrounding e2e work. Filing as a follow-up rather than bundling it into the test-coverage change.

## Repro

`packages/think/src/e2e-tests/workflow-recovery.test.ts` — the "recovery" case kills a structured workflow turn mid-stream and currently observes a terminal-but-non-`complete` workflow. Tightening that assertion to require `status === "complete"` with the validated output would reproduce the gap.

## Acceptance

- A structured workflow turn interrupted mid-stream recovers and completes with the validated structured output.
- `workflow-recovery.test.ts` is tightened to assert `complete` + correct output (removing the `NOTE (deferred)`).