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/mattermost-watcher-design.md

@@ -42,6 +42,8 @@ A categorized run must be treated differently:
 - the watcher must wait for that grouped sub-run to complete
 - then send that grouped sub-run's final Mattermost status
 - then continue watching for the next grouped sub-run
+- the watcher must remain alive while the parent categorized request or related child Cypress process is still active
+- one completed grouped sub-run must not be treated as proof that the parent categorized request is finished
 - the watcher must not wait until the very end to send one single parent-only post
 
 Evidence sources: