fix(installer): detect stale background-task pids

[patil2001]

Summary

bg_task_status used os.kill(pid,0) alone, which only proves some process
owns the number: a reaped and recycled pid is reported running, so
bg_task_wait spins to its 1200s timeout instead of completing. Record the
process start time at registration and only treat a live pid as the task
when its start time still matches.

AI Assistance

AI helped draft; I reviewed every hunk and ran the checks below.

Release Lane

• Stable hotfix targeting release/2.6.x
• Mainline change targeting main
• Next-minor work targeting the next feature/minor release
• Not sure; reviewer should help classify

Changed Surface

• Docs only
• Tests only
• Dashboard UI
• Dashboard API / host agent
• Installer / bootstrap / lifecycle
• Docker Compose / service manifests
• Model routing / Hermes / capabilities
• Network exposure / auth / proxy
• Dependencies / runtime wiring

Risk And Validation

• Risk level: Low
• Validation run:
  • git diff --check
  • Focused tests listed below

Commands/results:

�[0;31m\u2717�[0m live task not reported running (rc=1)

Operational Change Check

• This is an operational change and validation is recorded above.