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>

markitect/prompts/templates/models.py

@@ -46,6 +46,11 @@ class ContentMacro:
     raw_text: str = ""
     line_number: int = 0
 
+    def __post_init__(self) -> None:
+        """Auto-derive raw_text when built programmatically."""
+        if not self.raw_text:
+            self.raw_text = f"@{{{self.target}}}"
+
     def __str__(self) -> str:
         """String representation of macro."""
         params = ''.join(f"|{k}={v}" for k, v in self.parameters.items())