Integrate StdinSource into the main command so users can pipe logs:

  cat app.log | logpilot
  kubectl logs -f pod | logpilot
  docker logs -f container | logpilot

When stdin is a pipe, logpilot runs in streaming mode (no TUI),
auto-detecting log format (JSON, logfmt, plain) per line and
rendering styled output to stdout.

Features:
- Automatic pipe detection via IsPipe()
- Auto-format detection per line (mixed formats supported)
- Graceful signal handling (SIGINT/SIGTERM)
- Long line support (up to 1MB)
- Configurable backpressure (block or drop-oldest)

Closes #9