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

[github-actions]

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Changed directories: components.

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cc @LavenderQAQ

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Don't convert unknown child status to success

When cmd.Wait() returns ECHILD, the child has already been reaped and its exit status is unavailable; treating that as nil makes failed commands report success. In the exact class of environments this is guarding against (for example SIGCHLD ignored/inherited or another reaper consuming the child), sh -c 'exit 42' also produces ECHILD, so this path calls markCommandFinished(session, 0, "") and OnExecuteComplete instead of surfacing the failure, corrupting command results.

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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".