Start scheduler from Decodex App

[yvette-carlisle]

Summary

• make Decodex App fallback start ordinary decodex serve --listen-address 127.0.0.1:8912 instead of the hidden dev endpoint
• move the scheduler default into the Rust control plane as 15 seconds
• update README, app docs, runbook, and Decodex plugin skills so --dev is only for isolated local API development

Verification

• cargo make fmt
• cargo make lint-fix
• swift build --package-path apps/decodex-app
• cargo make test
• cargo make test-decodex-app-stage
• cargo run -q -p decodex --bin decodex -- serve --help
• semantic drift audit for App fallback, --dev, and scheduler interval wording