docs: realign MVP planning docs

Rewrite the MVP scope, gap analysis, and implementation backlog
so they reflect the agreed architecture and the current repo
state.

- make the consumer sidecar the primary SDS-facing boundary
- align discovery, pricing, and session semantics with the
  revised MVP decisions
- update gap and backlog status to reflect recent provider
  runtime and persistence work
- narrow remaining open questions to auth and observability

Author: juanmardefago