executor: document kill-goroutine safety and add goroutine-leak test

The pgid-kill goroutine in execOnce() uses a select with both ctx.Done()
and the killDone channel. Add a detailed comment explaining why the goroutine
cannot block indefinitely: the killDone arm fires unconditionally when
cmd.Wait() returns (whether the process exited naturally or was killed),
so the goroutine always exits before execOnce() returns.

Add TestExecOnce_NoGoroutineLeak_OnNaturalExit to verify this: it samples
runtime.NumGoroutine() before and after execOnce() with a no-op binary
("true") and a background context (never cancelled), asserting no net
goroutine growth.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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