fix(core): cleanup background bash processes so cli exits

[Octane0411]

Summary

Fix an agent hang seen in terminal-bench runs where tasks complete in transcript but Harbor stays at running agent and never reaches verifier.

Root Cause

Bash tool background mode (run_in_background) left child process/pipe handles attached to the Node/Bun event loop, and CLI had no shutdown cleanup for background processes. In tasks like configure-git-webserver, this can keep oas alive after final answer.

Changes

• packages/core/src/tools/bash.ts
  • unref background child process + stdio handles
  • add cleanupBackgroundProcesses() for best-effort termination on shutdown
• packages/cli/src/index.ts
  • call cleanupBackgroundProcesses() in finally before exit
• packages/core/src/tools/registry.ts and packages/core/src/index.ts
  • export cleanup helper
• packages/core/tests/tools/bash-enhanced.test.ts
  • add regression test for background cleanup

Validation

• bun test packages/core/tests/tools/bash-enhanced.test.ts
• bun test packages/core/tests/tools/kill-bash.test.ts packages/core/tests/tools/bash-output.test.ts
• bun run typecheck

All passed locally.