Add runnable smoke-test clients for each SDK

Each package gets a small example program that exercises the real
client: with no ORIGIN_SERVICE_KEY set it runs against a local mock
check-in server, otherwise it checks in against ORIGIN_URL/Origin's
public endpoint. Wired into CI (mock mode) as a regression check, and
documented per README.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -20,6 +20,7 @@ jobs:
       - run: go vet ./...
       - run: test -z "$(gofmt -l .)"
       - run: go test ./...
+      - run: go run ./examples/testclient
 
   js:
     runs-on: ubuntu-latest
@@ -37,6 +38,7 @@ jobs:
       - run: npm run typecheck
       - run: npm run build
       - run: npm test
+      - run: node examples/testclient.mjs
 
   rust:
     runs-on: ubuntu-latest
@@ -52,3 +54,4 @@ jobs:
       - run: cargo clippy --all-targets -- -D warnings
       - run: cargo build
       - run: cargo test
+      - run: cargo run --example testclient