fix(task): kill entire process group on BackgroundTaskRuntime dispose + test:noforce

[hernandez42]

Summary

Fix BackgroundTaskRuntime subprocess leak and add open-handle detection to test suite.

Changes

src/task/runtime/BackgroundTaskRuntime.ts

• On Unix, use process.kill(-child.pid, signal) to kill the entire process group (negative PID = process group), not just the shell process
• This catches detached bash sub-processes that would otherwise leak
• Windows still uses child.kill(signal) (TerminateProcess)

package.json

• test now runs without --test-force-exit (natural exit, exposes open handles)
• Added test:force as the old --test-force-exit mode
• Added test:noforce as explicit alias for natural-exit mode

Why this matters

--test-force-exit was masking open handles from background tasks, file watchers, and MCP connections. Running without it (npm test) now naturally exposes any real leaks:

npm test        # natural exit, detects open handles
npm run test:force   # legacy mode with --test-force-exit
npm run test:noforce # same as npm test

Verification

npm test
# Should pass AND exit cleanly without force-exit

Co-authored-by: Xuanji xuanji@ouvaton.org