feat(spaces): implement Phase 7 Composability

Implements space composition and inheritance features:
- SpaceReference model for space-to-space references (includes, extends, links_to, composed_of)
- Variable inheritance through parent chain with local override
- Config inheritance with source tracking
- Access control models (SpacePermission, SpaceRole, AccessLevel)
- InheritanceResolver for walking parent chains
- AccessControlService for permission management
- ComposableSpaceService integrating all composability features
- Circular reference detection for EXTENDS references
- SQLite repositories for references and permissions
- 57 comprehensive unit tests

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

markitect/spaces/composability/__init__.py

@@ -0,0 +1,49 @@
+"""
+Composability module for Information Spaces.
+
+Provides space-to-space references, variable/config inheritance,
+and basic access control for the Information Space system.
+"""
+
+from .models import (
+    SpaceReference,
+    SpaceReferenceType,
+    SpacePermission,
+    SpaceRole,
+    AccessLevel,
+    SpaceAccess,
+    InheritedVariable,
+    InheritedConfig,
+)
+from .service import (
+    ComposableSpaceService,
+    InheritanceResolver,
+    AccessControlService,
+)
+from .repository import (
+    ISpaceReferenceRepository,
+    IAccessControlRepository,
+    SqliteSpaceReferenceRepository,
+    SqliteAccessControlRepository,
+)
+
+__all__ = [
+    # Models
+    "SpaceReference",
+    "SpaceReferenceType",
+    "SpacePermission",
+    "SpaceRole",
+    "AccessLevel",
+    "SpaceAccess",
+    "InheritedVariable",
+    "InheritedConfig",
+    # Services
+    "ComposableSpaceService",
+    "InheritanceResolver",
+    "AccessControlService",
+    # Repositories
+    "ISpaceReferenceRepository",
+    "IAccessControlRepository",
+    "SqliteSpaceReferenceRepository",
+    "SqliteAccessControlRepository",
+]