Bound and cancel remote port discovery probes

[Luqueee]

Problem

In internal/discover/discover.go:121-145, Probe launches one goroutine per candidate and waits indefinitely for every Dial to return.

A stalled SSH channel or network path can block auto-discovery forever and prevent the daemon from responding to cancellation or shutdown.

Acceptance criteria

• Give each probe a bounded deadline.
• Allow the discovery cycle to be cancelled.
• Ensure all probe goroutines terminate after cancellation or timeout.
• Preserve the distinction between “probe unavailable” and “probe completed with no open ports”.
• Add tests for timeout and cancellation behavior.