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_builder.py

@@ -614,6 +614,101 @@ class TestDownloadTarget:
         assert (project_dir / "description.txt").exists()
 
 
+class TestContentTarget:
+    """Tests for content-type targets that write literal text."""
+
+    async def test_content_target_writes_file(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config = write_config({"targets": {"file.txt": {"content": "ABC"}}})
+        with patch("hokusai.builder._create_providers", return_value=_fake_providers()):
+            result = await run_build(config, project_dir, _PROJECT)
+
+        assert result.built == ["file.txt"]
+        assert (project_dir / "file.txt").read_text() == "ABC"
+
+    async def test_content_target_incremental_skip(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config = write_config({"targets": {"file.txt": {"content": "ABC"}}})
+        with patch("hokusai.builder._create_providers", return_value=_fake_providers()):
+            r1 = await run_build(config, project_dir, _PROJECT)
+            assert r1.built == ["file.txt"]
+
+            r2 = await run_build(config, project_dir, _PROJECT)
+            assert r2.skipped == ["file.txt"]
+            assert r2.built == []
+
+    async def test_content_target_rebuild_on_change(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config1 = write_config({"targets": {"file.txt": {"content": "ABC"}}})
+        with patch("hokusai.builder._create_providers", return_value=_fake_providers()):
+            r1 = await run_build(config1, project_dir, _PROJECT)
+            assert r1.built == ["file.txt"]
+
+            config2 = write_config({"targets": {"file.txt": {"content": "XYZ"}}})
+            r2 = await run_build(config2, project_dir, _PROJECT)
+            assert r2.built == ["file.txt"]
+            assert (project_dir / "file.txt").read_text() == "XYZ"
+
+    async def test_content_target_no_archive(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config1 = write_config(
+            {
+                "archive_folder": "archive",
+                "targets": {"file.txt": {"content": "v1"}},
+            }
+        )
+        with patch("hokusai.builder._create_providers", return_value=_fake_providers()):
+            r1 = await run_build(config1, project_dir, _PROJECT)
+            assert r1.built == ["file.txt"]
+
+            config2 = write_config(
+                {
+                    "archive_folder": "archive",
+                    "targets": {"file.txt": {"content": "v2"}},
+                }
+            )
+            r2 = await run_build(config2, project_dir, _PROJECT)
+            assert r2.built == ["file.txt"]
+
+        assert (project_dir / "file.txt").read_text() == "v2"
+        assert not (project_dir / "archive").exists()
+
+    async def test_content_target_as_dependency(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config = write_config(
+            {
+                "targets": {
+                    "data.txt": {"content": "some data"},
+                    "output.md": {
+                        "prompt": "Process the data",
+                        "inputs": ["data.txt"],
+                    },
+                }
+            }
+        )
+        with patch("hokusai.builder._create_providers", return_value=_fake_providers()):
+            result = await run_build(config, project_dir, _PROJECT)
+
+        assert "data.txt" in result.built
+        assert "output.md" in result.built
+        assert (project_dir / "data.txt").read_text() == "some data"
+
+    async def test_content_target_no_provider_needed(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config = write_config({"targets": {"file.txt": {"content": "ABC"}}})
+        with patch("hokusai.builder._create_providers", return_value=[]):
+            result = await run_build(config, project_dir, _PROJECT)
+
+        assert result.built == ["file.txt"]
+        assert result.failed == {}
+
+
 class TestPlaceholderPrompts:
     """Tests for prompt placeholder substitution in builds."""