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/watcher-service/README.md

@@ -65,6 +65,8 @@ Typical workflow:
    - detect each grouped sub-run in sequence from the parent run log
    - wait for that grouped sub-run to finish
    - send one Mattermost post for that grouped sub-run if it reached `COMPLETED` or `FAILED`
+   - keep the watcher alive while the parent categorized runner or related child Cypress process is still active
+   - do not treat one completed grouped sub-run as proof that the whole parent request is finished
    - continue to the next grouped sub-run
    - exit after the parent request reaches a terminal state