refactor(media): harden localRoots bypass (#16739)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 89dce69f50
Co-authored-by: steipete <58493+steipete@users.noreply.github.com>
Co-authored-by: steipete <58493+steipete@users.noreply.github.com>
Reviewed-by: @steipete

src/agents/workspace-dir.ts

@@ -0,0 +1,20 @@
+import path from "node:path";
+import { resolveUserPath } from "../utils.js";
+
+export function normalizeWorkspaceDir(workspaceDir?: string): string | null {
+  const trimmed = workspaceDir?.trim();
+  if (!trimmed) {
+    return null;
+  }
+  const expanded = trimmed.startsWith("~") ? resolveUserPath(trimmed) : trimmed;
+  const resolved = path.resolve(expanded);
+  // Refuse filesystem roots as "workspace" (too broad; almost always a bug).
+  if (resolved === path.parse(resolved).root) {
+    return null;
+  }
+  return resolved;
+}
+
+export function resolveWorkspaceRoot(workspaceDir?: string): string {
+  return normalizeWorkspaceDir(workspaceDir) ?? process.cwd();
+}