feat: !include YAML tag and folder-based workflow layout

- Add !include custom YAML tag for referencing external files (Fixes #582)
  - .md/.txt files included as strings
  - .json files parsed as JSON objects
  - .yaml/.yml files parsed as YAML objects
  - Paths resolved relative to the workflow YAML file

- Support foo/index.yaml as alternative to foo.yaml (Fixes #583)
  - Updated discoverProjectWorkflows(), findWorkflowInDir()
  - Updated workflowNameFromPath() for index.yaml detection
  - Flat files take priority over folder layout

- Added tests for both features

packages/cli-workflow/src/include.ts

@@ -0,0 +1,25 @@
+import { readFileSync } from "node:fs";
+import { extname, resolve } from "node:path";
+import { parse as parseYaml } from "yaml";
+
+/**
+ * Create a YAML customTags entry for !include that resolves file paths
+ * relative to the given base directory.
+ */
+export function createIncludeTag(baseDir: string) {
+  return {
+    tag: "!include",
+    resolve(str: string) {
+      const filePath = resolve(baseDir, str);
+      const content = readFileSync(filePath, "utf8");
+      const ext = extname(filePath).toLowerCase();
+      if (ext === ".json") {
+        return JSON.parse(content);
+      }
+      if (ext === ".yaml" || ext === ".yml") {
+        return parseYaml(content);
+      }
+      return content;
+    },
+  };
+}