test: make parallel process exits deterministic and side-effect free

[ORESoftware]

What changed

• Remove the unconditional global Git configuration mutation from every subprocess run.
• Validate that callers provide a non-empty command array before spawning.
• Replace the CHLD trap/double-wait pattern with explicit PID tracking and one deterministic result line per command.
• Wait for every peer before returning a single aggregate exit status.
• Propagate SIGINT and SIGTERM to active child process groups.
• Add three executable child-process journeys covering true concurrent startup, mixed success/failure completion, and global-config isolation plus empty-command rejection.
• Add a pinned, read-only GitHub Actions gate.

Why

A process helper should not mutate the user's global Git configuration. The previous asynchronous CHLD trap could also race with the final wait, making exit accounting dependent on signal timing rather than command results.

Validation

The concurrency test uses a two-process barrier that cannot succeed if commands run serially. The failure test proves a peer still finishes after another command exits 7, while the parent exits 1. The isolation test runs under a temporary HOME and verifies no .gitconfig is created.

[linear-code]

DEN-2097