Align Decodex docs and skills with runtime status

[yvette-carlisle]

Summary

• document Decodex App API-only listener and app snapshot/account APIs
• align Decodex manual/automation skills with API-only and skills/list preflight behavior
• update self-dogfood recovery guidance for skills/list diagnostics and first_error details

Verification

• git diff --check
• semantic drift audit packet reviewed against app-server and operator endpoint code
• cargo test -p decodex preflight --all-features
• cargo test -p decodex api_only --all-features