Update ATVM watcher for categorized sub-run posting

- update the watcher design and automation guide to treat --categorize as sequential ATVM sub-runs rather than one parent run with internal phases
- document that categorized runs should send one Mattermost status per completed grouped sub-run instead of one parent-only final post
- add a --categorize option to the watcher start helper so categorized mode is explicit in watcher startup
- update the watcher implementation to track categorized sub-runs separately, write per-subrun state, and post each completed grouped run once

atvm/watcher-service/start-atvm-run-watcher.sh

@@ -13,6 +13,7 @@ Options:
   --migration-style <text>
   --integration-plugin <text>
   --scope-description <text>
+  --categorize
   --state-root <path>   Default: /var/lib/atvm-run-watcher
 EOF
 }
@@ -23,6 +24,7 @@ CONFIG_FAMILY=""
 MIGRATION_STYLE=""
 INTEGRATION_PLUGIN=""
 SCOPE_DESCRIPTION=""
+WATCHER_CATEGORIZED="false"
 STATE_ROOT="/var/lib/atvm-run-watcher"
 
 while [[ $# -gt 0 ]]; do
@@ -33,6 +35,7 @@ while [[ $# -gt 0 ]]; do
     --migration-style) MIGRATION_STYLE="${2:-}"; shift 2 ;;
     --integration-plugin) INTEGRATION_PLUGIN="${2:-}"; shift 2 ;;
     --scope-description) SCOPE_DESCRIPTION="${2:-}"; shift 2 ;;
+    --categorize) WATCHER_CATEGORIZED="true"; shift ;;
     --state-root) STATE_ROOT="${2:-}"; shift 2 ;;
     -h|--help) usage; exit 0 ;;
     *) echo "Unknown argument: $1" >&2; usage >&2; exit 1 ;;
@@ -54,6 +57,7 @@ ATVM_WATCHER_CONFIG_FAMILY=${CONFIG_FAMILY@Q}
 ATVM_WATCHER_MIGRATION_STYLE=${MIGRATION_STYLE@Q}
 ATVM_WATCHER_INTEGRATION_PLUGIN=${INTEGRATION_PLUGIN@Q}
 ATVM_WATCHER_SCOPE_DESCRIPTION=${SCOPE_DESCRIPTION@Q}
+ATVM_WATCHER_CATEGORIZED=${WATCHER_CATEGORIZED@Q}
 EOF
 
 systemctl start "atvm-run-watcher@${BUILD_NAME}.service"