Rename serve dev endpoint

[yvette-carlisle]

Summary

• rename hidden serve endpoint mode from --api-only to --dev
• update Decodex App startup, operator snapshot connector state, and compatibility parsing
• document when --dev is appropriate versus ordinary scheduler serve or dashboard mock

Verification

• cargo make fmt
• cargo make lint-fix
• cargo make test
• swift build --package-path apps/decodex-app
• CLI smokes for serve --dev rejection of --interval/--config, hidden help behavior, and removed --api-only flag
• semantic drift audit for api-only/api_only residuals