Clarify manual fallback workflow in public documentation

[wxici]

Maintainer backlog item

WebMuse mentions manual construction package fallback, but the public documentation should explain when and why this fallback is used.

Why this matters

WebMuse should not fail completely when real Codex execution is unavailable, disabled, or unsafe.

Proposed scope

Document:

• what manual fallback means
• what files it exports
• when it should be used
• how it differs from real AI execution
• what safety checks still apply

Acceptance criteria

• Add a section to docs/ARCHITECTURE.md or a dedicated fallback document.
• Make clear that fallback does not run Codex automatically.
• Make clear that fallback is useful for review, handoff, and manual execution.