feat(prompts): implement Phase 1 - Foundation (FR-1)

Implement addressable artifacts with content-based identity and change detection.

Core Features:
- Artifact model with SHA-256 content digests
- ArtifactReference for cross-space addressing
- IArtifactRepository interface for pluggable storage
- SQLiteArtifactRepository implementation
- ArtifactService for high-level operations
- Content digest calculation utilities

Database:
- prompt_artifacts table with indexes
- Support for artifact metadata and types
- UNIQUE constraint on space_id+name

Tests (41 passing):
- 26 model tests (metadata, artifacts, references, digests)
- 15 repository tests (CRUD, queries, constraints)

Implements:
- FR-1.1: Unique addressability by name and ID
- FR-1.2: Content digest computation and storage
- FR-1.3: Cross-space artifact references

Files Created:
- markitect/prompts/models.py
- markitect/prompts/repositories/interfaces.py
- markitect/prompts/repositories/sqlite.py
- markitect/prompts/services/artifact_service.py
- migrations/prompts/001_create_artifacts_table.sql
- tests/unit/prompts/test_artifact_models.py
- tests/unit/prompts/test_artifact_repository.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

markitect/prompts/repositories/interfaces.py

@@ -0,0 +1,159 @@
+"""
+Repository interfaces for artifact persistence.
+
+Defines abstract interfaces for artifact storage, enabling
+pluggable storage backends.
+"""
+
+from abc import ABC, abstractmethod
+from typing import List, Optional
+from markitect.prompts.models import Artifact, ArtifactType
+
+
+class RepositoryError(Exception):
+    """Base exception for repository errors."""
+    pass
+
+
+class ArtifactNotFoundError(RepositoryError):
+    """Raised when an artifact cannot be found."""
+    pass
+
+
+class DuplicateArtifactError(RepositoryError):
+    """Raised when attempting to create an artifact with duplicate name."""
+    pass
+
+
+class IArtifactRepository(ABC):
+    """
+    Abstract interface for artifact persistence.
+
+    Implements FR-1: InformationSpace Addressability
+    Provides CRUD operations for artifacts with content digest tracking.
+    """
+
+    @abstractmethod
+    def create(self, artifact: Artifact) -> Artifact:
+        """
+        Persist a new artifact.
+
+        Args:
+            artifact: Artifact to create
+
+        Returns:
+            Created artifact
+
+        Raises:
+            DuplicateArtifactError: If artifact with same space_id+name exists
+            RepositoryError: On other persistence errors
+        """
+        pass
+
+    @abstractmethod
+    def get_by_id(self, artifact_id: str) -> Optional[Artifact]:
+        """
+        Retrieve artifact by ID.
+
+        Args:
+            artifact_id: Artifact identifier
+
+        Returns:
+            Artifact if found, None otherwise
+        """
+        pass
+
+    @abstractmethod
+    def get_by_name(self, space_id: str, name: str) -> Optional[Artifact]:
+        """
+        Retrieve artifact by space and name.
+
+        Implements FR-1.3: Cross-space artifact lookup
+
+        Args:
+            space_id: Space identifier
+            name: Artifact name
+
+        Returns:
+            Artifact if found, None otherwise
+        """
+        pass
+
+    @abstractmethod
+    def get_by_digest(self, content_digest: str) -> List[Artifact]:
+        """
+        Find artifacts with matching content digest.
+
+        Implements FR-1.2: Content digest queries
+
+        Args:
+            content_digest: SHA-256 digest to match
+
+        Returns:
+            List of artifacts with matching digest
+        """
+        pass
+
+    @abstractmethod
+    def list_by_space(
+        self,
+        space_id: str,
+        artifact_type: Optional[ArtifactType] = None,
+    ) -> List[Artifact]:
+        """
+        List all artifacts in a space.
+
+        Args:
+            space_id: Space identifier
+            artifact_type: Optional type filter
+
+        Returns:
+            List of artifacts in space
+        """
+        pass
+
+    @abstractmethod
+    def update(self, artifact: Artifact) -> Artifact:
+        """
+        Update an existing artifact.
+
+        Updates content digest and modified timestamp.
+
+        Args:
+            artifact: Artifact with updated data
+
+        Returns:
+            Updated artifact
+
+        Raises:
+            ArtifactNotFoundError: If artifact doesn't exist
+            RepositoryError: On other persistence errors
+        """
+        pass
+
+    @abstractmethod
+    def delete(self, artifact_id: str) -> bool:
+        """
+        Delete an artifact.
+
+        Args:
+            artifact_id: Artifact identifier
+
+        Returns:
+            True if deleted, False if not found
+        """
+        pass
+
+    @abstractmethod
+    def exists(self, space_id: str, name: str) -> bool:
+        """
+        Check if artifact exists.
+
+        Args:
+            space_id: Space identifier
+            name: Artifact name
+
+        Returns:
+            True if artifact exists
+        """
+        pass