guestd: Stream workload stdout and stderr

[t0gun]

Context

scaled already accepts raw bytes on the guest log vsock port and writes them to the VM workspace. After workload launch, guestd should connect workload output to that existing log channel.

Scope

• Capture child stdout.
• Capture child stderr.
• Stream both to the existing raw log vsock channel on port 10001.
• Preserve the current host-side log behavior; do not frame log bytes with the control protocol.
• Make failure behavior clear when the log channel is closed.

Acceptance Criteria

• Workload stdout appears in the host-side guestd.log file.
• Workload stderr appears in the host-side guestd.log file.
• Log streaming does not block child reaping indefinitely.
• CI remains green.

[t0gun]

Tracked by #9.