docs: create architecture diagram (Mermaid or Draw.io) showing component relationships

[charannyk06]

Priority: P3 — Low

Problem

ChainReview has a sophisticated multi-component architecture (VS Code extension, MCP server, 5 agents, 9+ tools, Cloudflare Workers API, Next.js dashboard, Supabase) but there is no visual architecture diagram. New contributors must piece together the architecture by reading code.

Solution

Add a docs/architecture.md with:

1. System Overview Diagram (Mermaid)

graph TB
    subgraph VSCode["VS Code Extension"]
        EXT[Extension Host]
        WV[Webview React UI]
        AUTH[Auth Provider]
        MCP_C[MCP Client]
    end
    
    subgraph Server["MCP Server (Node.js)"]
        ORCH[Orchestrator]
        subgraph Agents
            ARCH[Architecture Agent]
            SEC[Security Agent]
            BUGS[Bugs Agent]
            VAL[Validator Agent]
            EXP[Explainer Agent]
        end
        subgraph Tools
            REPO[repo tools]
            CODE[code tools]
            PATCH[patch tools]
            EXEC[exec tools]
            GRAPH[graph tools]
        end
        STORE[(SQLite Store)]
    end
    
    subgraph Cloud["Optional Cloud (Managed Mode)"]
        API[chainreview-api\nCloudflare Workers]
        DB[(Supabase PostgreSQL)]
        DASH[Dashboard\nNext.js]
    end
    
    ANTHROPIC[Anthropic API\nClaude claude-opus-4-6]

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2. Review Run Sequence Diagram

Show the flow: user triggers review → extension → MCP server → agents run in parallel → findings returned → webview displays.

3. Data Flow Diagram

What data goes where, what stays local vs. what goes to the cloud.

4. Auth Flow Diagram

Supabase OAuth → JWT → API key management flow.

Acceptance Criteria

• System overview diagram in docs/architecture.md
• Review run sequence diagram
• Data flow diagram (local vs. cloud)
• Linked from README.md