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/core/__init__.py

@@ -0,0 +1,50 @@
+"""
+Core infrastructure modules for MarkiTect.
+
+This package contains the fundamental building blocks:
+- Parser: Markdown to AST conversion
+- Serializer: AST to Markdown serialization
+- DocumentManager: Document ingestion and management
+- Workspace: Workspace and project management
+"""
+
+from .parser import parse_markdown_to_ast
+from .serializer import ASTSerializer
+from .document_manager import DocumentManager, CleanDocumentManager
+from .workspace import (
+    WorkspaceManager,
+    WorkspaceTemplate,
+    TemplateMetadata,
+    TemplateResult,
+    WorkspaceCreationResult,
+    ProjectResult,
+    SyncResult,
+    BackupResult,
+    RestoreResult,
+    WorkspaceState,
+    ConflictInfo,
+    MergeResult,
+)
+
+__all__ = [
+    # Parser
+    "parse_markdown_to_ast",
+    # Serializer
+    "ASTSerializer",
+    # Document Manager
+    "DocumentManager",
+    "CleanDocumentManager",
+    # Workspace
+    "WorkspaceManager",
+    "WorkspaceTemplate",
+    "TemplateMetadata",
+    "TemplateResult",
+    "WorkspaceCreationResult",
+    "ProjectResult",
+    "SyncResult",
+    "BackupResult",
+    "RestoreResult",
+    "WorkspaceState",
+    "ConflictInfo",
+    "MergeResult",
+]