Keep categorized ATVM watcher alive until parent run finishes

- update the watcher to treat categorized parent-run activity as the authoritative signal for whether the overall request is still running
- prevent the watcher from exiting early just because one categorized grouped sub-run completed and wrote artifacts
- document that categorized watcher instances must remain alive between grouped runs until the parent request has actually gone inactive past the grace window
- update the ATVM guide, watcher design, and install docs to reflect the stricter categorized parent-run completion rule

atvm/docs/automation/guide.md

@@ -45,6 +45,8 @@ Run ATVM CMC automation tests on the designated automation VM without unintended
 - Treat `approve with watcher` as approval to run and also start the per-run watcher service for that build.
 - When `--categorize` is used with watcher enabled, treat the watcher as a sequential grouped-run watcher:
   - it must post one final Mattermost status per completed categorized group/sub-run
+  - it must stay active between grouped sub-runs while the parent categorized request is still running
+  - it must not stop after the first grouped run simply because one grouped run completed
   - it must not wait and replace those with one single parent-only post
 - After execution, report immediate success/failure only.
 - Do not actively monitor completion unless explicitly requested.