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/models.py

@@ -89,6 +89,7 @@ class Artifact:
     artifact_type: ArtifactType
     content_digest: str
     content_size: int = 0
+    content: str = ""
     metadata: ArtifactMetadata = field(default_factory=ArtifactMetadata)
     created_at: datetime = field(default_factory=datetime.utcnow)
     updated_at: datetime = field(default_factory=datetime.utcnow)
@@ -126,6 +127,7 @@ class Artifact:
             artifact_type=artifact_type,
             content_digest=content_digest,
             content_size=content_size,
+            content=content,
             metadata=metadata or ArtifactMetadata(),
         )
 
@@ -136,6 +138,7 @@ class Artifact:
         Args:
             new_content: New content string
         """
+        self.content = new_content
         self.content_digest = calculate_content_digest(new_content)
         self.content_size = len(new_content.encode('utf-8'))
         self.updated_at = datetime.utcnow()
@@ -161,6 +164,7 @@ class Artifact:
             "artifact_type": self.artifact_type.value,
             "content_digest": self.content_digest,
             "content_size": self.content_size,
+            "content": self.content,
             "metadata": self.metadata.to_dict(),
             "created_at": self.created_at.isoformat(),
             "updated_at": self.updated_at.isoformat(),
@@ -176,6 +180,7 @@ class Artifact:
             artifact_type=ArtifactType(data["artifact_type"]),
             content_digest=data["content_digest"],
             content_size=data.get("content_size", 0),
+            content=data.get("content", ""),
             metadata=ArtifactMetadata.from_dict(data.get("metadata", {})),
             created_at=datetime.fromisoformat(data["created_at"]),
             updated_at=datetime.fromisoformat(data["updated_at"]),