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_template_models.py

@@ -32,6 +32,23 @@ class TestContentMacro:
         )
         assert macro.parameters == {"language": "python", "framework": "fastapi"}
 
+    def test_programmatic_macro_gets_auto_derived_raw_text(self):
+        """Test that programmatically-built macro gets auto-derived raw_text."""
+        macro = ContentMacro(
+            kind=MacroKind.REQUIRED,
+            target="glossary",
+        )
+        assert macro.raw_text == "@{glossary}"
+
+    def test_explicit_raw_text_not_overridden(self):
+        """Test that explicit raw_text is preserved."""
+        macro = ContentMacro(
+            kind=MacroKind.REQUIRED,
+            target="glossary",
+            raw_text="{{require:glossary}}",
+        )
+        assert macro.raw_text == "{{require:glossary}}"
+
     def test_macro_str_representation(self):
         """Test string representation."""
         macro = ContentMacro(