fix: reap exited function workers

[omercnet]

Summary

• centralize runtime worker process ownership so every started worker is reaped exactly once
• make worker stop, cleanup, and context cancellation share the managed lifecycle
• close worker log pipes after a bounded idle drain when descendants inherit stdout or stderr

Closes #6971

Verification

• go test ./pkg/process ./pkg/runtime
• go test -race ./pkg/process ./pkg/runtime
• bun run build:platform
• bun run build:cli
• bun run test:cli