Reset reused watcher state before starting a new ATVM run

- update the watcher start helper to stop any stale watcher instance for the same requested parent build name and remove its old state directory before starting fresh
- document that reused parent build names must not inherit stale cancelled, posted, state.json, or subruns state from older runs
- update the watcher install and design docs so the controller workflow explicitly treats stale reused-build-name state as part of startup cleanup

atvm/docs/automation/mattermost-watcher-design.md

@@ -144,6 +144,10 @@ Possible contents:
 - one cancellation marker per parent run id
 - optional lock file to prevent multiple watcher instances from racing
 
+When the same requested parent build name is reused for a new run:
+- the watcher start workflow must clear old watcher state for that requested build name before starting
+- stale `cancelled.marker`, `posted.marker`, `state.json`, and `subruns/` contents must not be allowed to affect the new run
+
 ## Recommended Operator Workflow
 Normal completion workflow:
 1. ATVM run starts.