chore: remove MVP positioning and align plan defaults (#15)

## Summary
- remove MVP wording from repository docs and guidance
- rename the system plan document and update references to it
- align the default subscription plan code/name with product wording
- document hard subscription expiry with no grace period

## Verification
- docker build -f infra/docker/web.Dockerfile .
- docker build -f infra/docker/migrate.Dockerfile .

Co-authored-by: sirily <sirily@git.shararam.party>
Reviewed-on: #15

AGENTS.md

@@ -5,7 +5,7 @@ This file governs the whole repository unless a deeper `AGENTS.md` overrides it.
 
 ## Source of truth
 Read these files before making architectural changes:
-- `docs/plan/mvp-system-plan.md`
+- `docs/plan/system-plan.md`
 - `docs/architecture/system-overview.md`
 - `docs/architecture/repository-layout.md`
 - `docs/ops/deployment.md`
@@ -37,7 +37,7 @@ This repository is a TypeScript monorepo for `nproxy`, a crypto-subscription ima
 - Keep provider-specific HTTP code out of domain services.
 - Model user request attempts separately from provider-key attempts.
 - Preserve the chosen deployment target: single VPS with Docker Compose.
-- Preserve the chosen billing model for MVP: manual crypto invoice renewal.
+- Preserve the chosen billing model: manual crypto invoice renewal.
 - Preserve the chosen quota display contract: user-facing approximate buckets only.
 
 ## Required follow-up when changing key areas