fix: detect race condition when file is modified during user confirma…

[vyagh]

Summary

Fixes #24

The EditTool reads a file during user confirmation but doesn't detect if the file is modified externally before writing. This causes silent data loss when users edit the same file while the agent waits for approval.

Changes

• Add EDIT_FILE_MODIFIED_DURING_WAIT error type
• Capture file mtime when read during shouldConfirmExecute()
• Validate mtime before write in execute() - abort with actionable error if changed
• Add unit test for race condition detection

How it works

1. When shouldConfirmExecute() reads the file, we store its modification time
2. At the start of execute(), before any operations, we check if the file's current mtime differs
3. If changed, return an error instructing the agent to re-read the file

[Copilot]

Pull request overview

This PR addresses issue #24 by adding race condition detection to the EditTool. It prevents silent data loss when a file is modified externally during the user confirmation period.

Key Changes:

• Added EDIT_FILE_MODIFIED_DURING_WAIT error type for race condition detection
• Implemented mtime-based validation by capturing file modification time during confirmation and checking it before write
• Added test coverage for the race condition scenario

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
packages/core/src/tools/tool-error.ts	Added new error type EDIT_FILE_MODIFIED_DURING_WAIT for file modification detection
packages/core/src/tools/edit.ts	Added readMtimeMs field to track file modification time, implemented mtime capture in shouldConfirmExecute() and validation in execute(), plus minor formatting changes
packages/core/src/tools/edit.test.ts	Added test case to verify race condition detection between confirmation and execution

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