Execution output now streams in real time via SSE.

Previously, /exec blocked until the command finished and returned all output as a single JSON response. From v0.7, output streams line by line as it's produced — no more waiting.

### Changes - POST /v1/sandboxes/{id}/exec now returns text/event-stream - Dep install remains silent — only your code's output streams - Test mode streams pytest output line by line - SDK exec() updated to consume the stream, returns str