fix(relay): add root nixpacks.toml to install scripts/ deps on Railway (#1760)

Railway builds from repo root (root_dir=""), so scripts/nixpacks.toml is
skipped. Root packages are installed via npm ci but scripts/package.json
deps (@anthropic-ai/sdk, fast-xml-parser, etc.) never land in
/app/node_modules, causing ERR_MODULE_NOT_FOUND at runtime.

Add a root nixpacks.toml that:
- Preserves existing scripts/nixpacks.toml settings (curl, NODE_OPTIONS)
- Adds `npm install --prefix scripts` to the build phase so all
  scripts/package.json deps are installed alongside root deps

nixpacks.toml

@@ -0,0 +1,17 @@
+# Railway relay build config (root_dir="" — builds from repo root).
+# Promotes scripts/nixpacks.toml settings here since nixpacks only reads
+# config from the build root. Adds scripts/ dep install so that packages
+# in scripts/package.json (e.g. @anthropic-ai/sdk, fast-xml-parser) are
+# available at runtime when node scripts/ais-relay.cjs is started.
+
+[phases.setup]
+aptPkgs = ["curl"]
+
+[variables]
+NODE_OPTIONS = "--dns-result-order=ipv4first"
+
+[phases.install]
+cmds = ["npm ci"]
+
+[phases.build]
+cmds = ["npm install", "npm install --prefix scripts"]