[Bug] Fable 5 safeguard `[reasoning_extraction]` false-positives on a one-word greeting ("Hi")

### Summary

Fable 5 returns `API Error: Fable 5's safeguards flagged this message` with `Details: [reasoning_extraction]` on a message whose user text is the single word `Hi`.

The `[reasoning_extraction]` classifier guards against prompts attempting to extract the model's chain of thought. Nothing in the conversation does that — the first and only user turn was a greeting.

### Error

```
API Error: Fable 5's safeguards flagged this message (https://www.anthropic.com/legal/aup).
This sometimes happens with safe, normal conversations. Claude Code can't respond to this message with Fable 5.

Double press esc to edit your last message, or try a different model with /model.

Details: `[reasoning_extraction]`

Request ID: req_011CeAG9pE2qTzUcm6Vbm3aD
```

### Steps to reproduce

1. Start Claude Code in a project that has a global `~/.claude/CLAUDE.md`, a project `CLAUDE.md`, a memory index file, and a large set of installed skills + MCP servers.
2. `/model fable`
3. Send `Hi`.
4. Request is rejected with the error above.

The same session context works normally on Opus 5 — only Fable 5 rejects it, which points at the additional Fable-only safeguards rather than at the conversation.

### Expected

A one-word greeting is not a reasoning-extraction attempt and should not be blocked.

### Observed / notes

- Retrying reproduces the block rather than passing intermittently.
- No user-authored instruction in the loaded context asks the model to reveal, dump, or reproduce its reasoning or system prompt.
- The practical effect is that Fable 5 is unusable in this project regardless of what is typed, because the rejection is driven by the loaded context, not the message.

### Environment

- Claude Code: 2.1.234
- Model: Fable 5 (`claude-fable-5`)
- Platform: macOS 26.3 (Apple Silicon)
- Request ID: `req_011CeAG9pE2qTzUcm6Vbm3aD`

### Related

Existing Fable-safeguard false-positive reports, all dual-use/security-code flavored rather than `[reasoning_extraction]`: #85303, #73577, #86856, #86804.

Reproduce in Claude Code 2.1.234 with the global and project CLAUDE.md files, memory index, installed skills, and MCP servers described in the report. Compare the same session using Fable 5 and Opus 5, and use the reported reasoning_extraction error and request ID to trace the failure. Done means a one-word greeting is accepted without the safeguard error.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.