Auto-generate Mermaid sequence diagrams in PR reviews

[haasonsaas]

Problem

Greptile and CodeRabbit auto-generate Mermaid sequence diagrams showing call flow for changed functions. These are highly shared by engineering teams and function as organic marketing. DiffScope has the smart_review_diagram config option and the symbol graph to construct these, but:

1. It's off by default
2. It's not wired into the GitHub PR comment flow
3. The symbol graph data isn't used to generate accurate call-flow diagrams

Proposed Solution

1. Enable diagram generation by default in smart review mode
2. Use symbol graph data to generate accurate sequence/flow diagrams:
   • Call flow: API Handler → Service → Repository → Database
   • Data flow: How data transforms through the changed code path
   • Dependency graph: What modules are affected by this change
3. Embed Mermaid diagrams in PR comments — GitHub natively renders Mermaid in markdown
4. Collapsible by default in the summary comment (use <details> tags)
5. Generate diagrams only when meaningful — skip for trivial changes (typos, config, etc.)

Example Output

sequenceDiagram
    participant Client
    participant AuthMiddleware
    participant UserController
    participant UserService
    participant PostgreSQL
    
    Client->>AuthMiddleware: POST /api/users (changed)
    AuthMiddleware->>UserController: validateToken() 
    UserController->>UserService: createUser(dto) (changed)
    UserService->>PostgreSQL: INSERT users (changed)
    PostgreSQL-->>UserService: User record
    UserService-->>UserController: UserResponse

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Priority

Tier 2 — Differentiator and marketing asset. These diagrams are highly shareable.