feat: add prompt placeholder substitution with {filename} syntax

tests/test_builder.py

@@ -13,7 +13,6 @@ from hokusai.builder import (
     _collect_all_deps,  # pyright: ignore[reportPrivateUsage]
     _collect_dep_files,  # pyright: ignore[reportPrivateUsage]
     _collect_extra_params,  # pyright: ignore[reportPrivateUsage]
-    _resolve_prompt,  # pyright: ignore[reportPrivateUsage]
     run_build,
 )
 from hokusai.config import GenerateTargetConfig, ProjectConfig
@@ -112,21 +111,6 @@ def _fake_providers() -> list[Provider]:
     return [FakeTextProvider(), FakeImageProvider()]
 
 
-class TestResolvePrompt:
-    """Test prompt resolution (file vs inline)."""
-
-    def test_inline_prompt(self, project_dir: Path) -> None:
-        assert _resolve_prompt("Just a string", project_dir) == "Just a string"
-
-    def test_file_prompt(self, project_dir: Path, prompt_file: Path) -> None:
-        result = _resolve_prompt(prompt_file.name, project_dir)
-        assert result == "This prompt comes from a file"
-
-    def test_nonexistent_file_treated_as_inline(self, project_dir: Path) -> None:
-        result = _resolve_prompt("no_such_file.txt", project_dir)
-        assert result == "no_such_file.txt"
-
-
 class TestCollectHelpers:
     """Test dependency collection helpers."""
 
@@ -568,3 +552,47 @@ class TestDownloadTarget:
         assert "description.txt" in result.built
         assert (project_dir / "fish.png").read_bytes() == b"fake fish image"
         assert (project_dir / "description.txt").exists()
+
+
+class TestPlaceholderPrompts:
+    """Tests for prompt placeholder substitution in builds."""
+
+    async def test_placeholder_in_prompt_triggers_rebuild(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        _ = (project_dir / "style.txt").write_text("impressionist")
+        config = write_config(
+            {
+                "targets": {
+                    "out.txt": {"prompt": "Paint in {style.txt} style"},
+                }
+            }
+        )
+        with patch("hokusai.builder._create_providers", return_value=_fake_providers()):
+            r1 = await run_build(config, project_dir, _PROJECT)
+            assert r1.built == ["out.txt"]
+            content1 = (project_dir / "out.txt").read_text()
+            assert "impressionist" in content1
+
+            # Change the placeholder file
+            _ = (project_dir / "style.txt").write_text("cubist")
+            r2 = await run_build(config, project_dir, _PROJECT)
+            assert r2.built == ["out.txt"]
+
+    async def test_placeholder_deps_in_collect_all(
+        self, write_config: WriteConfig
+    ) -> None:
+        config = write_config(
+            {
+                "targets": {
+                    "out.txt": {
+                        "prompt": "Use {a.txt} and {b.txt}",
+                        "inputs": ["c.txt"],
+                    },
+                }
+            }
+        )
+        deps = _collect_all_deps("out.txt", config)
+        assert "a.txt" in deps
+        assert "b.txt" in deps
+        assert "c.txt" in deps