Diff preview tool for dry-run edits

[tjdoomer]

Summary

• Dry-run edit tool returning unified diff without modifying files
• Validates file exists, old_string found, and old_string is unique
• Uses existing diff package dependency (createPatch)
• Registered as Kind.Read — no confirmation needed

Test plan

• All 1873 tests pass
• Type-check clean

Summary by CodeRabbit

New Features

• Added a file preview tool that performs dry-run edits and generates unified diffs, enabling review of proposed file changes without modifying the original files.
• Supports configurable context lines for flexible diff visualization.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 5a8b6e86-7031-4af6-b57e-f1706e2acd27

📥 Commits

Reviewing files that changed from the base of the PR and between 732221b and ccb196a.

📒 Files selected for processing (3)

• docs/roadmap/11-diff-preview.md
• packages/core/src/config/config.ts
• packages/core/src/tools/diffPreview.ts

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📝 Walkthrough

Walkthrough

This PR introduces a new diff_preview tool that performs a dry-run file edit operation. It reads a target file, validates that the old string exists exactly once, computes an in-memory replacement, and generates a unified diff output without modifying the disk. The tool is registered in the core tool registry and documented in the roadmap.

Changes

Cohort / File(s)	Summary
Documentation docs/roadmap/11-diff-preview.md	Roadmap document describing the diff_preview tool concept, validation behavior, unified diff generation with configurable context lines, and example output.
Core Tool Implementation packages/core/src/tools/diffPreview.ts	New DiffPreviewTool class that accepts file_path, old_string, new_string, and optional context_lines parameters. Implements file reading, exact-match validation (rejecting zero or multiple matches), in-memory string replacement, and unified diff generation via the diff package.
Tool Registration packages/core/src/config/config.ts	Registers DiffPreviewTool as an enabled core tool in the Config.createToolRegistry() method alongside existing core tools.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 A preview tool hops into view,
Dry-runs your changes without a "moo!"
No files are harmed—just diffs to see,
Perfect for checking before you commit, tee-hee! ✨

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/diff-preview

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