[shell-operator] fix: track child PIDs to avoid zombie reaper race

[fuldaxxx]

Overview

Adds a process registry to the executor package so that the PID-1 zombie reaper can distinguish between children it owns and children spawned by the executor (hooks, scripts). This prevents a race condition where SIGCHLD handling reaps a child that cmd.Wait still expects to collect, causing "signal: child exited already" errors and lost exit codes.

What this PR does / why we need it

When shell-operator runs as PID 1 in a container, its zombie reaper loop calls waitpid(-1, ...) on every SIGCHLD. This can steal a child that was just started by the executor - cmd.Start() returns, the child exits, SIGCHLD fires, and the reaper reaps the process before cmd.Wait() gets to it. The result is a spurious exec: signal: child exited already or missing exit-code propagation.

This PR introduces:

• processRegistry (registry.go) — a thread-safe, package-internal map that tracks PIDs of processes started by the executor. Exposes a read-only ProcessTracker interface via Tracker() for external consumers (zombie reaper) and package-private registerPID/unregisterPID helpers for the executor methods.
• Modified Run, Output, and RunAndLogLines in executor.go — split cmd.Run() / cmd.Output() into explicit Start → registerPID → Wait → unregisterPID sequences so the child PID is
  tracked for the entire lifetime between fork and reap.
• Comprehensive tests (executor_test.go) — covers registry basics, double-unregister safety, concurrent access, and integration with Output / RunAndLogLines (including failed-start scenarios).

The zombie reaper can now call executor.Tracker().IsActive(pid) to skip PIDs that the executor will reap itself.

[Copilot]

Pull request overview

Adds executor-owned PID tracking so a PID-1 zombie reaper can avoid reaping processes that the executor will Wait() on, preventing “child exited already” / lost-exit-code races when running as PID 1.

Changes:

• Introduced a package-internal, thread-safe PID registry with a read-only ProcessTracker interface (Tracker()).
• Updated command execution paths (Run, Output, RunAndLogLines) to use Start → registerPID → Wait → unregisterPID to keep child PIDs tracked during their lifetime.
• Added unit/integration-style tests around the registry and executor integration.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
pkg/executor/registry.go	Adds the PID registry + ProcessTracker interface and global tracker access.
pkg/executor/executor.go	Registers/unregisters child PIDs around Start/Wait for Run/Output/RunAndLogLines.
pkg/executor/executor_test.go	Adds registry tests and new executor/registry integration tests.

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