Fix: Allow empty result string in reviewDiff validation

[Felipe-RA]

Important

Solves #7

Problem

The reviewDiff function in reviewer.ts rejects valid review completions when the AI agent returns an empty string as the result. This occurs with large diffs (130K+ chars) where the agent uses only toolbox functions (leave_inline_comment, leave_general_comment) without additional text output.

Error observed:

[Amp] Started thread: T-06b3f3b3-a708-08371dc31a64
[Amp] Review completed successfully
Error starting thread: Error: Amp review failed

Root Cause

Line 81 in src/review/reviewer.ts:

if (!threadId || !result) throw new Error('Amp review failed');

• Empty string is falsy in JavaScript, so !result evaluates to true
• This causes the check to fail even when the review succeeded
• Comments are collected via the JSONL file, not the result string
• The result field is never used after being returned (verified in process-review.ts)

Solution

Changed the validation to only check for threadId:

if (!threadId) throw new Error('No thread ID received from Amp');

This allows empty result strings (which should be valid when AI uses only tools to write comments) while still catching legitimate errors (no thread created).

Testing

• TypeScript type-check passes
• Build succeeds
• Verified result field is never accessed in process-review.ts
• Reproduced the bug locally with tool-only AI responses
• Confirmed fix resolves the issue when triggered on the local docker

Impact

• Safe: result field is never used after this, so removing the check doesn't break anything
• Backward compatible: Still returns result in response object