feat: add Nix flake with package and NixOS module

Add a Nix flake that builds the Paseo daemon (server + CLI) and
provides a NixOS module for declarative deployment.

Package (nix/package.nix):
- Builds relay, server, and CLI workspaces
- Skips onnxruntime-node install script (sandbox-incompatible)
- Rebuilds only node-pty for native terminal support
- Source filter excludes app/website/desktop workspaces

NixOS module (nix/module.nix):
- Systemd service with configurable user, port, listen address
- allowedHosts for DNS rebinding protection
- relay.enable to toggle remote access via app.paseo.sh
- inheritUserEnvironment to expose user tools (git, ssh) to agents
- openFirewall and extra environment variables

ci: add Nix hash maintenance scripts and workflows

scripts/fix-lockfile.mjs:
  Adds missing resolved/integrity fields to package-lock.json for
  workspace-local overrides. Idempotent, uses `npm view`.

scripts/update-nix.sh:
  Runs fix-lockfile.mjs, prefetches deps, computes NAR hash, and
  updates npmDepsHash in nix/package.nix. Supports --check for CI.

.github/workflows/nix-build.yml:
  Builds the Nix package on push/PR and verifies the lockfile and
  hash are up to date.

.github/workflows/fix-nix-hash.yml:
  Auto-fixes lockfile signatures and Nix hash on dependabot PRs.

fix: update npmDepsHash after upstream sync

nix: allowlist workspace symlinks instead of blocklist

Prevents build failures when upstream adds new workspace packages.

scripts/fix-lockfile.mjs

@@ -0,0 +1,66 @@
+#!/usr/bin/env node
+// Ensure all workspace-local node_modules entries in package-lock.json have
+// `resolved` and `integrity` fields. npm omits these for workspace-hoisted
+// overrides, but tools like Nix's offline `npm ci` require them.
+//
+// This script is idempotent — running it on an already-complete lockfile
+// is a no-op. Run it after `npm install` when the lockfile changes.
+//
+// Usage:
+//   node scripts/fix-lockfile.mjs
+//   node scripts/fix-lockfile.mjs path/to/package-lock.json
+
+import fs from "fs";
+import { execSync } from "child_process";
+
+const lockPath = process.argv[2] || "package-lock.json";
+const lock = JSON.parse(fs.readFileSync(lockPath, "utf8"));
+
+// Collect workspace package roots (local packages, not from npm)
+const workspaceRoots = new Set();
+for (const [key, val] of Object.entries(lock.packages || {})) {
+  if (val.link) {
+    workspaceRoots.add(val.resolved || key);
+  }
+}
+
+let fixed = 0;
+
+for (const [key, val] of Object.entries(lock.packages || {})) {
+  if (
+    !key || // root package
+    key.startsWith("node_modules/") || // top-level (already has resolved)
+    val.link || // workspace link entry
+    (val.resolved && val.integrity) || // already complete
+    !val.version || // no version to look up
+    workspaceRoots.has(key) // workspace package root (local, not on npm)
+  )
+    continue;
+
+  const pkgName = key.replace(/.*node_modules\//, "");
+  const version = val.version;
+
+  try {
+    const info = JSON.parse(
+      execSync(`npm view ${pkgName}@${version} --json dist`, {
+        encoding: "utf8",
+        stdio: ["pipe", "pipe", "pipe"],
+      })
+    );
+    if (info.tarball && info.integrity) {
+      val.resolved = info.tarball;
+      val.integrity = info.integrity;
+      fixed++;
+    }
+  } catch {
+    console.error(`Warning: could not fetch info for ${pkgName}@${version}`);
+  }
+}
+
+fs.writeFileSync(lockPath, JSON.stringify(lock, null, 2) + "\n");
+
+if (fixed > 0) {
+  console.log(`Fixed ${fixed} lockfile entries with missing resolved/integrity`);
+} else {
+  console.log("Lockfile is already complete");
+}

scripts/update-nix.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+# Fix workspace-local lockfile entries and update the Nix dependency hash.
+# Requires: node, npm, nix (with prefetch-npm-deps from nixpkgs)
+#
+# Usage:
+#   ./scripts/update-nix.sh          # fix lockfile + update hash
+#   ./scripts/update-nix.sh --check  # verify everything is up to date (CI mode)
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+ROOT_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+LOCK_FILE="$ROOT_DIR/package-lock.json"
+PACKAGE_NIX="$ROOT_DIR/nix/package.nix"
+
+CHECK_MODE=false
+if [[ "${1:-}" == "--check" ]]; then
+  CHECK_MODE=true
+fi
+
+# 1. Fix lockfile (add resolved/integrity for workspace-local entries)
+echo "Fixing lockfile..."
+node "$SCRIPT_DIR/fix-lockfile.mjs" "$LOCK_FILE"
+
+# 2. Prefetch deps and compute hash
+echo "Prefetching npm dependencies..."
+TMPDIR_DEPS="$(mktemp -d)"
+trap "rm -rf $TMPDIR_DEPS" EXIT
+
+prefetch-npm-deps "$LOCK_FILE" "$TMPDIR_DEPS/deps" 2>/dev/null
+NEW_HASH="$(nix hash path "$TMPDIR_DEPS/deps")"
+echo "Computed hash: $NEW_HASH"
+
+# 3. Read current hash
+CURRENT_HASH="$(grep 'npmDepsHash' "$PACKAGE_NIX" | sed 's/.*"\(.*\)".*/\1/')"
+
+if [[ "$NEW_HASH" == "$CURRENT_HASH" ]]; then
+  echo "Hash is already up to date."
+else
+  if $CHECK_MODE; then
+    echo "ERROR: npmDepsHash is stale."
+    echo "  current: $CURRENT_HASH"
+    echo "  correct: $NEW_HASH"
+    echo "Run ./scripts/update-nix.sh to fix."
+    exit 1
+  fi
+
+  echo "Updating npmDepsHash in nix/package.nix..."
+  sed -i.bak "s|npmDepsHash = \".*\"|npmDepsHash = \"$NEW_HASH\"|" "$PACKAGE_NIX"
+  rm -f "$PACKAGE_NIX.bak"
+  echo "Updated: $CURRENT_HASH -> $NEW_HASH"
+fi