Add config store runtime config spec

[ChristianPavilonis]

Summary

• add a concrete spec for moving application config from build-time embedding to runtime config store loading
• define production vs development config behavior, including Fastly/Viceroy local-dev projection into a simulated config store
• scope the doc to config-store architecture only, with hashing included and signatures/tooling UX deferred

Testing

• not run (docs-only change)