Compaction failed: received empty response from model (3x retry, manual /compact on Opus 4.6)

### Describe the bug

Running `/compact` manually in a short session (< 30 turns) on Claude Opus 4.6 produces three consecutive failures:

```
X Compaction Failed: Error: Compaction failed: received empty response from model
X Compaction Failed: Error: Compaction failed: received empty response from model
X Compaction Failed: Error: Compaction failed: received empty response from model
```

The session remained usable — after one additional turn (wasting a premium request), a subsequent `/compact` succeeded normally.

### Affected version

1.0.34

### Steps to reproduce the behavior

1. Start a session using Claude Opus 4.6.
2. Work through a task (commit pushed, environment reloaded mid-session — MCP servers, extensions, hooks, agents, skills all reloaded at the same time).
3. Run `/compact` manually.
4. Observe three consecutive `Compaction failed: received empty response from model` errors.

### Expected behavior

`/compact` should succeed, or at minimum degrade gracefully (e.g., retry with a different summarization strategy rather than repeating the same failing call three times).

### Additional context

- **Model**: Claude Opus 4.6
- **Session length**: Short (< 30 turns)
- **Trigger**: Manual `/compact` command
- **Environment**: Windows, VS Code Insiders connected, 2 custom instructions, 1 extension, 1 hook, 6 agents, 14 skills
- **MCP servers**: 0 connected at the time of failure (had just reloaded)
- **Recovery**: Session continued normally after one additional turn, then `/compact` worked — but this wasted a premium request on the recovery turn.

The environment reload (`MCP Servers reloaded: 0 servers connected`, `mermaid-render extension loaded`, `Environment loaded: ...`) occurred immediately before the compaction failures, which may be relevant — the compaction model may have received an unusual message history containing reload notifications.

### Related issues

- #2500 — Compaction killed the session (different symptom but same area)
- #2553 — Compaction fails after model switching (different root cause)
- #2333 — Request to toggle off auto-compaction (workaround request)

首先，在按描述重新加载环境后，使用 Claude Opus 4.6 重现手动 /compact 失败，然后跟踪 compaction 请求及其 retry 处理。检查空模型响应是如何处理的；当该场景不再产生连续三次失败，并且在无法完成 compaction 时能够优雅降级，即表示完成。

由索引模型根据 Issue 内容生成。