feat: modularize flattener tool into separate components with improved project root detection (#417)

tools/flattener/projectRoot.js

@@ -0,0 +1,45 @@
+const fs = require("fs-extra");
+const path = require("node:path");
+
+/**
+ * Attempt to find the project root by walking up from startDir
+ * Looks for common project markers like .git, package.json, pyproject.toml, etc.
+ * @param {string} startDir
+ * @returns {Promise<string|null>} project root directory or null if not found
+ */
+async function findProjectRoot(startDir) {
+  try {
+    let dir = path.resolve(startDir);
+    const root = path.parse(dir).root;
+    const markers = [
+      ".git",
+      "package.json",
+      "pnpm-workspace.yaml",
+      "yarn.lock",
+      "pnpm-lock.yaml",
+      "pyproject.toml",
+      "requirements.txt",
+      "go.mod",
+      "Cargo.toml",
+      "composer.json",
+      ".hg",
+      ".svn",
+    ];
+
+    while (true) {
+      const exists = await Promise.all(
+        markers.map((m) => fs.pathExists(path.join(dir, m))),
+      );
+      if (exists.some(Boolean)) {
+        return dir;
+      }
+      if (dir === root) break;
+      dir = path.dirname(dir);
+    }
+    return null;
+  } catch {
+    return null;
+  }
+}
+
+module.exports = { findProjectRoot };