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/__init__.py

@@ -0,0 +1,20 @@
+"""
+Repository layer for Prompt Dependency Resolution.
+
+This package provides abstract interfaces and concrete implementations
+for persisting prompts system entities.
+"""
+
+from markitect.prompts.repositories.interfaces import (
+    IArtifactRepository,
+    RepositoryError,
+    ArtifactNotFoundError,
+    DuplicateArtifactError,
+)
+
+__all__ = [
+    "IArtifactRepository",
+    "RepositoryError",
+    "ArtifactNotFoundError",
+    "DuplicateArtifactError",
+]