Kill the forked agent run if it outlives runtimeout

[silug]

Short description

The daemon forks a child for each catalog run and waited on it with a bare Process.waitpid2. The child enforces runtimeout on its own run, but nothing guaranteed that it would actually exit afterwards: a stranded thread or hung subprocess left the parent blocked forever and the agent stopped checking in until the service was restarted.

Poll the child with WNOHANG and, once runtimeout plus a grace period for sending the report has elapsed, SIGKILL it and reap it so the daemon carries on with its schedule. A runtimeout of 0 keeps the old blocking behaviour.

Fixes part of #485

Generated by Claude Code

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)