Cancel-aware streaming on shutdown signal

[bug-ops]

Parent: #355

Wrap stream consumption loops with tokio::select! on shutdown signal:

• process_response_streaming() — chunk stream loop
• process_response_native_tools() — tool loop iterations

After early break: flush accumulated chunks, persist partial response.

Reuse: shutdown_signal() at crates/zeph-core/src/agent/mod.rs:803