fix(execd): avoid global signal to fix false command failures

[LavenderQAQ]

Summary

Fixes #1041.

execd could report a successfully-executed command as a failure (CommandExecError) when cmd.Wait() returned a spurious ECHILD ("waitid: no child processes").
The root cause was global signal handling in runCommand / runBackgroundCommand: they called signal.Notify(signals) with no signal list (capturing ALL signals, including SIGCHLD and SIGURG) and defer signal.Reset() (a process-global reset). This interfered with the Go runtime's use of SIGCHLD/SIGURG (child reaping and async preemption) and raced across concurrent/sequential commands, occasionally leaving Wait() unable to reap its own child.

This change:

1. Replaces signal.Notify(signals) + signal.Reset() with signal.Notify(signals, forwardSignals...) + signal.Stop(signals), so only an explicit set of signals is forwarded and cleanup is scoped to this channel instead of resetting global handlers.
2. Ignores spurious ECHILD from cmd.Wait() (child already reaped) so a command that ran to completion is reported as success instead of a false failure.

Testing

• Unit tests
• e2e / manual verification

Breaking Changes

• None

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

[chatgpt-codex-connector]

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[LavenderQAQ]

As mentioned at the end of #1041, I need some time to observe whether this patch really resolves the issue. Once this problem no longer occurs, I will convert this PR to "ready".

[Pangjiping]

LGTM overall — root cause analysis is solid and the fix is correct. Scoping signal.Notify to an explicit list + using signal.Stop instead of the global signal.Reset cleanly eliminates the SIGCHLD/SIGURG interference and the cross-command race.

Ready to merge once you mark it as ready for review 👍

[LavenderQAQ]

@Pangjiping Thank you for your review. After this fix, I no longer observed the error "waitid: no child processes", so I marked this PR as ready.

[Pangjiping]

@Pangjiping Thank you for your review. After this fix, I no longer observed the error "waitid: no child processes", so I marked this PR as ready.

Thanks for this. Great Jobs 🎉

[Pangjiping]

LGTM