fix(prompts): fix three infrastructure bugs in prompt dependency resolution

- ContentMacro: add __post_init__ to auto-derive raw_text when built
  programmatically, preventing str.replace("", X) corruption
- MacroParser: add @{target} shorthand syntax support mapped to REQUIRED kind,
  updating parse, has_macros, count_macros, and find_macro_positions
- Artifact: store content in model and SQLite DB, replace resolver placeholder
  with actual artifact content, add migration for existing databases

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

tests/unit/prompts/test_artifact_repository.py

@@ -259,3 +259,32 @@ class TestSQLiteArtifactRepository:
         assert retrieved.metadata.author == "test-author"
         assert retrieved.metadata.version == "1.0.0"
         assert retrieved.metadata.custom == {"key": "value"}
+
+    def test_content_round_trip(self, repository):
+        """Test that artifact content survives store and retrieve."""
+        original_content = "# Test Content\n\nThis is the full content."
+        artifact = Artifact.create(
+            space_id="test-space",
+            name="content-test",
+            content=original_content,
+        )
+
+        repository.create(artifact)
+        retrieved = repository.get_by_id(artifact.id)
+
+        assert retrieved.content == original_content
+
+    def test_content_persisted_after_update(self, repository):
+        """Test that updated content is persisted."""
+        artifact = Artifact.create(
+            space_id="test-space",
+            name="update-test",
+            content="Original content",
+        )
+        repository.create(artifact)
+
+        artifact.update_content("Updated content")
+        repository.update(artifact)
+
+        retrieved = repository.get_by_id(artifact.id)
+        assert retrieved.content == "Updated content"