Add sidecar-nostr-relay example and fix openwrt CI path

Add a complete example running a strfry Nostr relay exclusively over
the FIPS mesh using the sidecar pattern. Includes Docker Compose
stack, network isolation via iptables, .fips DNS resolution, nak CLI,
build script with cross-compilation support, and documentation.

Also fix the package-openwrt.yml workflow path to match the
openwrt → openwrt-ipk directory rename.

.github/workflows/package-openwrt.yml

@@ -96,7 +96,7 @@ jobs:
         env:
           PKG_VERSION: ${{ steps.version.outputs.version }}
           LLVM_STRIP: llvm-strip
-        run: ./packaging/openwrt/build-ipk.sh --arch ${{ matrix.build_arch }}
+        run: ./packaging/openwrt-ipk/build-ipk.sh --arch ${{ matrix.build_arch }}
 
       - name: Upload artifact
         uses: actions/upload-artifact@v4