GitHub suggestion blocks for one-click fix application

[haasonsaas]

Problem

When DiffScope posts review comments on PRs, it uses plain text comments. Every major competitor (CodeRabbit, Bito, Greptile, GitHub Copilot) now posts inline GitHub suggestion blocks that render an "Apply suggestion" button, letting developers accept fixes with one click.

This is consistently cited as the #1 developer experience gap in competitive reviews.

Proposed Solution

When posting inline PR comments that include a suggested fix, format the suggestion using GitHub's suggestion syntax:

```suggestion
fixed code here
```

This requires:

1. Detecting when a review comment includes a code fix/suggestion
2. Mapping the suggestion to the exact diff lines in the PR
3. Formatting the GitHub comment body with the suggestion block syntax
4. Handling multi-line suggestions correctly

Competitive Context

Tool	One-click fix	Implementation
CodeRabbit	✅	GitHub suggestion blocks + IDE apply
GitHub Copilot	✅	Agentic — spawns Copilot agent to apply
Bito	✅	GitHub suggestion blocks
Greptile	✅	GitHub suggestion blocks
Qodo Merge	✅	GitHub suggestion blocks
DiffScope	❌	Plain text comments only

Priority

Tier 1 — Table stakes. This is now expected by developers evaluating AI code review tools.

[haasonsaas]

Implementation Details (from codebase analysis)

Current State

• Comment struct has code_suggestion: Option<CodeSuggestion> with { original, suggested, explanation, diff } fields
• src/commands/pr.rs posts comments via gh api repos/.../pulls/{pr}/comments
• Comments are posted as plain text — no GitHub suggestion block formatting

Files to Modify

• src/commands/pr.rs — modify build_github_comment_body() (or equivalent) to format code_suggestion as GitHub suggestion blocks:

  ```suggestion
  <suggested code>
  ```
  

• Need to map suggestion line ranges to GitHub's diff position format (GitHub requires specific line/start_line params for multi-line suggestions)

Key Challenge

GitHub suggestion blocks require exact line mapping to the PR diff. The line parameter in the review comment API must point to the exact diff position. Multi-line suggestions need start_line + line range.

Estimated Effort

Small-medium — formatting is simple, line mapping needs care.