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>

migrations/prompts/001_create_artifacts_table.sql

@@ -0,0 +1,34 @@
+-- Migration 001: Create prompt artifacts table
+-- Implements FR-1: InformationSpace Addressability
+-- Date: 2026-02-08
+
+-- Prompt artifacts table for content-based addressing
+CREATE TABLE IF NOT EXISTS prompt_artifacts (
+    id TEXT PRIMARY KEY,
+    space_id TEXT NOT NULL,
+    name TEXT NOT NULL,
+    artifact_type TEXT NOT NULL,
+    content_digest TEXT NOT NULL,
+    content_size INTEGER DEFAULT 0,
+    metadata JSON,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    UNIQUE(space_id, name)
+);
+
+-- Indexes for performance
+CREATE INDEX IF NOT EXISTS idx_artifacts_space ON prompt_artifacts(space_id);
+CREATE INDEX IF NOT EXISTS idx_artifacts_digest ON prompt_artifacts(content_digest);
+CREATE INDEX IF NOT EXISTS idx_artifacts_type ON prompt_artifacts(artifact_type);
+CREATE INDEX IF NOT EXISTS idx_artifacts_name ON prompt_artifacts(space_id, name);
+
+-- Comments (for documentation)
+-- prompt_artifacts.id: Unique UUID identifier for the artifact
+-- prompt_artifacts.space_id: Reference to information space containing this artifact
+-- prompt_artifacts.name: Human-readable name, unique within space
+-- prompt_artifacts.artifact_type: Classification: content, template, generated, schema, config
+-- prompt_artifacts.content_digest: SHA-256 hash of content for change detection
+-- prompt_artifacts.content_size: Size of content in bytes
+-- prompt_artifacts.metadata: JSON extensible metadata (description, tags, author, version, custom)
+-- prompt_artifacts.created_at: Artifact creation timestamp
+-- prompt_artifacts.updated_at: Last modification timestamp