[PHASE 2] Implement Comment Deduplication (Update vs. Create) #607

async function findExistingReviewerComment(octokit, owner, repo, prNumber) {
  const { data: comments } = await octokit.rest.issues.listComments({
    owner,
    repo,
    issue_number: prNumber,
  });
  
  return comments.find(c => c.body.includes('<!-- reviewer-agent-summary -->'));
}

const existingComment = await findExistingReviewerComment(octokit, owner, repo, pr.number);

const summary = buildSummaryMarkdown() + '\\n<!-- reviewer-agent-summary -->';

if (existingComment) {
  await octokit.rest.issues.updateComment({
    owner,
    repo,
    comment_id: existingComment.id,
    body: summary
  });
  core.info('Updated existing review comment');
} else {
  await octokit.rest.issues.createComment({
    owner,
    repo,
    issue_number: pr.number,
    body: summary
  });
  core.info('Created new review comment');
}

test('creates comment on first run', async () => {
  const octokit = mockOctokit({
    listComments: jest.fn().mockResolvedValue({ data: [] })
  });
  
  await run(context, { octokit });
  
  expect(octokit.rest.issues.createComment).toHaveBeenCalled();
  expect(octokit.rest.issues.updateComment).not.toHaveBeenCalled();
});

test('updates comment on second run', async () => {
  const existingComment = {
    id: 123,
    body: '<!-- reviewer-agent-summary -->...'
  };
  
  const octokit = mockOctokit({
    listComments: jest.fn().mockResolvedValue({ 
      data: [existingComment] 
    })
  });
  
  await run(context, { octokit });
  
  expect(octokit.rest.issues.updateComment).toHaveBeenCalledWith(
    expect.objectContaining({ comment_id: 123 })
  );
  expect(octokit.rest.issues.createComment).not.toHaveBeenCalled();
});