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/resolver/resolver.py

@@ -143,9 +143,7 @@ class PromptResolver:
             )
 
             if artifact:
-                # Found! Get content (would need to load from storage in real impl)
-                # For now, we'll use a placeholder
-                content = f"[Content of {artifact.name} from {space_id}]"
+                content = artifact.content
 
                 resolved = ResolvedMacro(
                     macro=macro,