Stream stdout/stderr to the logger for every transport mode

[m-messer]

Summary

• Every worker's stderr is now streamed line-by-line to the logger as it's produced (via a new worker.LogPipe helper), instead of being buffered silently and only surfaced as a single blob when the process exits non-zero. This applies uniformly to all transport modes (rpc/stdio, rpc/ipc, rpc/http, rpc/ws, rpc/tcp, file) since they all share ProcessWorker.Start.
• RPC transports other than stdio (ipc, http, ws, tcp) now also pipe and log the worker's stdout, which was previously discarded to /dev/null since nothing read it. stdio transport intentionally still leaves stdout untouched, since it's the literal JSON-RPC channel and logging it would corrupt the protocol framing.
• The file adapter's stdout handling was refactored to use the same shared LogPipe helper instead of its own ad hoc buffer-and-dump-at-EOF logic, so behavior (and diagnostics) are consistent across adapters.

Test plan

• go build ./...
• go vet ./...
• go test ./internal/execution/worker/... ./internal/execution/supervisor/... (including TestWorker_CapturesStderr, confirming the ExitEvent.Stderr contract is unchanged)
• Manual smoke test: run a worker under each RpcConfig.Transport and the file adapter with --log-level debug, confirm stdout/stderr lines appear in logs (except stdout under stdio transport, which must remain protocol-only)