refactor(sandbox): share bind parsing and host-path policy checks

src/agents/sandbox/host-paths.ts

@@ -0,0 +1,47 @@
+import { existsSync, realpathSync } from "node:fs";
+import { posix } from "node:path";
+
+/**
+ * Normalize a POSIX host path: resolve `.`, `..`, collapse `//`, strip trailing `/`.
+ */
+export function normalizeSandboxHostPath(raw: string): string {
+  const trimmed = raw.trim();
+  return posix.normalize(trimmed).replace(/\/+$/, "") || "/";
+}
+
+/**
+ * Resolve a path through the deepest existing ancestor so parent symlinks are honored
+ * even when the final source leaf does not exist yet.
+ */
+export function resolveSandboxHostPathViaExistingAncestor(sourcePath: string): string {
+  if (!sourcePath.startsWith("/")) {
+    return sourcePath;
+  }
+
+  const normalized = normalizeSandboxHostPath(sourcePath);
+  let current = normalized;
+  const missingSegments: string[] = [];
+
+  while (current !== "/" && !existsSync(current)) {
+    missingSegments.unshift(posix.basename(current));
+    const parent = posix.dirname(current);
+    if (parent === current) {
+      break;
+    }
+    current = parent;
+  }
+
+  if (!existsSync(current)) {
+    return normalized;
+  }
+
+  try {
+    const resolvedAncestor = normalizeSandboxHostPath(realpathSync.native(current));
+    if (missingSegments.length === 0) {
+      return resolvedAncestor;
+    }
+    return normalizeSandboxHostPath(posix.join(resolvedAncestor, ...missingSegments));
+  } catch {
+    return normalized;
+  }
+}