Investigate spawnImpl consistency in TUI controller

[SorraTheOrc]

Non-blocking follow-up from PR #1199 review for WL-0MN8QL3AO0008ZW3. Review src/tui/controller.ts for direct child_process spawn usage in command execution paths (for example around runNextWorkItems) and ensure testable process execution goes through the injected spawnImpl abstraction where appropriate. Add or update tests to verify mocked process behavior.\n\nrelated-to:WL-0MN8QL3AO0008ZW3