feat: add content target type for writing literal text to files

Content targets write a string directly to the output file without
invoking any AI provider. They don't require API keys and are not
archived when overwritten.

Example usage in .hokusai.yaml:
  file.txt:
    content: ABC

tests/test_config.py

@@ -7,7 +7,7 @@ from pathlib import Path
 import pytest
 import yaml
 
-from hokusai.config import GenerateTargetConfig, load_config
+from hokusai.config import ContentTargetConfig, GenerateTargetConfig, load_config
 
 
 class TestLoadConfig:
@@ -83,3 +83,14 @@ class TestLoadConfig:
 
         with pytest.raises(Exception):
             _ = load_config(config_path)
+
+    def test_content_target(self, project_dir: Path) -> None:
+        config_path = project_dir / "test.hokusai.yaml"
+        _ = config_path.write_text(
+            yaml.dump({"targets": {"file.txt": {"content": "ABC"}}})
+        )
+        config = load_config(config_path)
+
+        target = config.targets["file.txt"]
+        assert isinstance(target, ContentTargetConfig)
+        assert target.content == "ABC"