Add architecture package and application skeleton

docs/backlog/01_execution_backlog.md

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+# Execution Backlog
+
+## Package A: Architecture Freeze
+
+- [x] runtime ADR
+- [x] storage ADR
+- [x] event stream ADR
+- [x] worker transport ADR
+- [x] policy evaluation ADR
+- [x] event taxonomy ADR
+- [x] model fallback ADR
+- [x] domain model
+- [x] state machines
+- [x] storage schema draft
+- [x] internal interfaces
+- [x] error model
+- [x] execution model
+
+## Package B: Core Hardening
+
+- [ ] graph scheduler service
+- [ ] node runner abstraction
+- [ ] reviewer contract
+- [ ] finalizer contract
+- [ ] resume/cancel use cases
+- [ ] idempotency policy
+- [ ] retry policy matrix
+
+## Package C: Persistence
+
+- [ ] SQLAlchemy models
+- [ ] repositories for PostgreSQL/SQLite
+- [ ] migrations
+- [ ] artifact metadata persistence
+- [ ] event store queries with pagination
+
+## Package D: Integrations
+
+- [ ] model router providers
+- [ ] MCP transport adapter
+- [ ] worker WebSocket gateway
+- [ ] heartbeat monitor
+- [ ] capability-aware dispatch
+
+## Package E: Delivery
+
+- [ ] typed worker registration schema
+- [ ] task event DTO normalization
+- [ ] confirmation cards
+- [ ] progress cards
+- [ ] artifact cards
+- [ ] error cards
+
+## Package F: Validation
+
+- [ ] contract tests
+- [ ] scenario tests
+- [ ] smoke suite
+- [ ] static checks in CI
+