feat(spaces): implement Phase 0-1 of Information Space Service

Phase 0 - Project Organization:
- Create docs/PROJECT_STRUCTURE.md documenting codebase layout
- Create markitect/core/ with parser, serializer, document_manager, workspace
- Create markitect/schema/ consolidating 6 schema_*.py modules
- Create markitect/storage/ with database module
- Maintain backward compatibility via re-exports from original locations
- Add docs/roadmap/information-space-service/ with README and WORKPLAN

Phase 1 - Foundation (Weeks 1-3):
- Week 1: Core domain models (InformationSpace, SpaceDocument, SpaceConfig,
  SpaceMetadata, SpaceVariable, TransclusionReference, SpaceStatus)
- Week 2: Repository layer with interfaces (ISpaceRepository,
  IDocumentAssociationRepository, IVariableRepository, IReferenceRepository)
  and SQLite implementations with foreign key cascade deletes
- Week 3: SpaceService orchestration layer with full CRUD, document,
  variable, and reference tracking operations

Test coverage: 124 tests (25 model + 63 repository + 36 integration)

Capabilities delivered:
- CAP-001: InformationSpace entity with lifecycle management
- CAP-002: SpaceRepository CRUD with SQLite backing
- CAP-003: Document-Space associations with path-based organization
- CAP-004: Space metadata and configuration schemas
- CAP-005: Database schema with migrations

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

markitect/spaces/repositories/__init__.py

@@ -0,0 +1,38 @@
+"""
+Repository layer for Information Spaces.
+
+This package provides data access abstractions:
+- ISpaceRepository: Abstract interface for space CRUD operations
+- SqliteSpaceRepository: SQLite implementation
+- IDocumentAssociationRepository: Document-space association storage
+- IVariableRepository: Space variable storage
+- IReferenceRepository: Transclusion reference tracking
+"""
+
+from .interfaces import (
+    ISpaceRepository,
+    IDocumentAssociationRepository,
+    IVariableRepository,
+    IReferenceRepository,
+)
+from .sqlite import (
+    SqliteSpaceRepository,
+    SqliteDocumentRepository,
+    SqliteVariableRepository,
+    SqliteReferenceRepository,
+    initialize_space_tables,
+)
+
+__all__ = [
+    # Interfaces
+    "ISpaceRepository",
+    "IDocumentAssociationRepository",
+    "IVariableRepository",
+    "IReferenceRepository",
+    # SQLite implementations
+    "SqliteSpaceRepository",
+    "SqliteDocumentRepository",
+    "SqliteVariableRepository",
+    "SqliteReferenceRepository",
+    "initialize_space_tables",
+]