`edit` tool: "Could not find oldString" failure — model context drift

## Problem

The `edit` tool fails with:

```
Could not find oldString in the file. It must match exactly, including whitespace, indentation, and line endings.
```

236 occurrences in a single day. This is the second most common tool failure after `webfetch`.

## Root cause

The model's in-context representation of a file drifts from the actual file content. This happens when:

1. **External processes** (formatters, linters) modify files after the model reads them — partially addressed by #450
2. **Multiple edits** to the same file where the model uses stale oldString from before previous edits
3. **Context compaction** drops or summarizes file content, leaving the model with an outdated view
4. **Model hallucination** — generating oldString that was never in the file

## Impact

- Each failed edit wastes a tool call
- Model often retries with the same incorrect oldString, creating short failure bursts
- The existing doom loop detector (threshold=3) catches identical inputs but not semantically similar ones

## Suggested investigation

1. Analyze whether failures correlate with files that were recently edited (context drift) vs. files never read (hallucination)
2. Consider including a snippet of the actual file content in the error response so the model can self-correct (similar approach to #450 for stale files)
3. Evaluate whether the fuzzy matchers (LineTrimmedReplacer, WhitespaceNormalizedReplacer, etc.) are catching enough cases or if thresholds need tuning