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

hokusai/builder.py

@@ -12,7 +12,12 @@ from pathlib import Path
 import httpx
 
 from hokusai.archive import archive_file
-from hokusai.config import DownloadTargetConfig, GenerateTargetConfig, ProjectConfig
+from hokusai.config import (
+    ContentTargetConfig,
+    DownloadTargetConfig,
+    GenerateTargetConfig,
+    ProjectConfig,
+)
 from hokusai.graph import build_graph, get_build_order, get_subgraph_for_target
 from hokusai.prompt import extract_placeholder_files, resolve_prompt
 from hokusai.providers import Provider
@@ -151,12 +156,17 @@ async def _build_single_target(
     # Ensure parent directories exist for targets in subfolders.
     output_path.parent.mkdir(parents=True, exist_ok=True)
 
+    target_cfg = config.targets[target_name]
+
+    # Content targets write literal text — no archiving, no provider needed.
+    if isinstance(target_cfg, ContentTargetConfig):
+        _ = output_path.write_text(target_cfg.content)
+        return
+
     # Archive the existing artifact before overwriting.
     if config.archive_folder is not None:
         _ = archive_file(output_path, project_dir, config.archive_folder)
 
-    target_cfg = config.targets[target_name]
-
     if isinstance(target_cfg, DownloadTargetConfig):
         await _download_target(target_name, target_cfg, project_dir)
         return
@@ -267,6 +277,14 @@ def _is_dirty(
     """Check if a target needs rebuilding."""
     target_cfg = config.targets[target_name]
 
+    if isinstance(target_cfg, ContentTargetConfig):
+        return is_target_dirty(
+            target_name,
+            content=target_cfg.content,
+            state=state,
+            project_dir=project_dir,
+        )
+
     if isinstance(target_cfg, DownloadTargetConfig):
         return is_target_dirty(
             target_name,
@@ -300,7 +318,7 @@ def _has_provider(
 ) -> bool:
     """Check that the required provider is available; record failure if not."""
     target_cfg = config.targets[target_name]
-    if isinstance(target_cfg, DownloadTargetConfig):
+    if isinstance(target_cfg, (DownloadTargetConfig, ContentTargetConfig)):
         return True
     model_info = resolve_model(target_name, target_cfg, config.defaults)
     if model_info.name not in provider_index:
@@ -345,7 +363,14 @@ def _process_outcomes(
         else:
             target_cfg = config.targets[name]
 
-            if isinstance(target_cfg, DownloadTargetConfig):
+            if isinstance(target_cfg, ContentTargetConfig):
+                record_target_state(
+                    name,
+                    content=target_cfg.content,
+                    state=state,
+                    project_dir=project_dir,
+                )
+            elif isinstance(target_cfg, DownloadTargetConfig):
                 record_target_state(
                     name,
                     download=target_cfg.download,