Add plan linter module

src/linter.ts

@@ -0,0 +1,11 @@
+// Plan linter
+import { parse } from "./parser";
+
+export function lint(plan: string) {
+  const lines = parse(plan);
+  const warnings: string[] = [];
+  for (const line of lines) {
+    if (line.includes("ASAP")) warnings.push("Ambiguous urgency: ASAP");
+  }
+  return warnings;
+}