feat: add content targets and loop expansion for target templates

Content targets write literal text to files via 'content:' field,
without requiring an AI provider or API keys. They are not archived
when overwritten.

Loop expansion allows defining 'loops:' at the top level with named
lists of values. Targets with [var] in their name are expanded via
cartesian product. Variables are substituted in all string fields.
Explicit targets override expanded ones. Escaping: \[var] -> [var].
Expansion happens at config load time so the rest of the system
(builder, graph, state) sees only expanded targets.

tests/test_builder.py

@@ -709,6 +709,68 @@ class TestContentTarget:
         assert result.failed == {}
 
 
+class TestLoopExpansion:
+    """End-to-end tests for loop-expanded targets in builds."""
+
+    async def test_loop_content_targets_build(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config = write_config(
+            {
+                "loops": {"n": ["1", "2", "3"]},
+                "targets": {"file-[n].txt": {"content": "Value [n]"}},
+            }
+        )
+        with patch("hokusai.builder._create_providers", return_value=_fake_providers()):
+            result = await run_build(config, project_dir, _PROJECT)
+
+        assert set(result.built) == {"file-1.txt", "file-2.txt", "file-3.txt"}
+        assert (project_dir / "file-1.txt").read_text() == "Value 1"
+        assert (project_dir / "file-2.txt").read_text() == "Value 2"
+        assert (project_dir / "file-3.txt").read_text() == "Value 3"
+
+    async def test_loop_incremental_skip(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config = write_config(
+            {
+                "loops": {"n": ["1", "2"]},
+                "targets": {"file-[n].txt": {"content": "Value [n]"}},
+            }
+        )
+        with patch("hokusai.builder._create_providers", return_value=_fake_providers()):
+            r1 = await run_build(config, project_dir, _PROJECT)
+            assert len(r1.built) == 2
+
+            r2 = await run_build(config, project_dir, _PROJECT)
+            assert r2.built == []
+            assert set(r2.skipped) == {"file-1.txt", "file-2.txt"}
+
+    async def test_loop_with_dependency_chain(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config = write_config(
+            {
+                "loops": {"id": ["a", "b"]},
+                "targets": {
+                    "data-[id].txt": {"content": "Data for [id]"},
+                    "summary-[id].txt": {
+                        "prompt": "Summarize [id]",
+                        "inputs": ["data-[id].txt"],
+                    },
+                },
+            }
+        )
+        with patch("hokusai.builder._create_providers", return_value=_fake_providers()):
+            result = await run_build(config, project_dir, _PROJECT)
+
+        assert "data-a.txt" in result.built
+        assert "data-b.txt" in result.built
+        assert "summary-a.txt" in result.built
+        assert "summary-b.txt" in result.built
+        assert result.failed == {}
+
+
 class TestPlaceholderPrompts:
     """Tests for prompt placeholder substitution in builds."""