Stream Warm Pool Workspace Updates

Server-Sent Events stream that delivers live warm-hook stdout/stderr plus status transitions for a single warm pool workspace. The monolith proxies output from the warming sandbox's engine; output is only available while the sandbox is running (i.e. status is warming). Once the workspace reaches snapshotting the sandbox is suspended and the stream stops emitting output events — only status transitions and the terminal complete event still flow. The stream auto-closes on terminal status (ready, failed, expired, assigned). Events emitted: output (appended chunk), status (transition between warming and snapshotting), complete (terminal with final workspace record), and error.