Add a README architecture diagram for the WebMuse safety-first workflow

[wxici]

Maintainer backlog item

WebMuse currently explains the safety-first workflow in text, but the README would be easier to review if it included a small architecture diagram.

Why this matters

The project is early alpha. A clear diagram helps outside reviewers understand the intended flow without assuming that real Codex execution or website generation is already enabled.

Proposed scope

Add a lightweight Mermaid diagram showing:

• reference insight
• observation package
• construction package
• Codex task package
• sandbox validation
• dry-run
• proof-check
• approval gate
• controlled execution or manual fallback
• validation
• export

Acceptance criteria

• README includes a Mermaid diagram.
• The diagram does not imply that real Codex execution is currently enabled.
• The diagram keeps WebMuse positioned as a safety-first workflow tool.