feat: add prompt placeholder substitution with {filename} syntax

tests/test_graph.py

@@ -194,3 +194,30 @@ class TestGetSubgraphForTarget:
 
         assert sub.has_edge("input.txt", "summary.md")
         assert sub.has_edge("summary.md", "final.txt")
+
+    def test_placeholder_files_in_graph(
+        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"},
+                }
+            }
+        )
+        graph = build_graph(config, project_dir)
+        assert graph.has_edge("style.txt", "out.txt")
+
+    def test_escaped_placeholder_not_in_graph(
+        self, project_dir: Path, write_config: WriteConfig
+    ) -> None:
+        config = write_config(
+            {
+                "targets": {
+                    "out.txt": {"prompt": "Literal \\{not_a_dep.txt}"},
+                }
+            }
+        )
+        graph = build_graph(config, project_dir)
+        assert "not_a_dep.txt" not in graph.nodes