Add execution-only workflow worker API

[snamber]

Summary

• add the public execution-only Go workflow Worker API used by tilebox runner start
• share task registration and execution semantics with native TaskRunner
• implement lifecycle-safe WorkerService serving over the existing private Unix-socket protocol
• add real grpc-go and Connect-over-gRPC integration coverage, shutdown/cancellation/race/security tests, and a complete release-worker example
• honor TILEBOX_API_URL consistently and clean up Worker-owned telemetry

Validation

• go test ./...
• go test -race ./workflows/v1
• go build ./...
• golangci-lint run ./...
• Windows amd64 cross-build
• Darwin arm64 cross-build
• git diff --check

No generated protobuf/API files changed.