Show placement candidates and each node's answer in indexer_placement, add indexer_placement_reread, and drop a group's candidates when its head changes no placement file, so a missed message or a revert is recoverable by tool (issue 1126, ADR 0559 §3, §7, §8)

modules/mesh-delivery-indexer/module.json

@@ -3,7 +3,8 @@
   "version": "1",
   "slug": "indexer",
   "tools": [
-    "indexer_take_in"
+    "indexer_take_in",
+    "indexer_placement_reread"
   ],
   "emits": [
     "placement-changed"
@@ -34,7 +35,9 @@
   "invokes": [
     "gitea.gitea_commit_trees",
     "gitea.gitea_contains",
-    "gitea.gitea_get_file"
+    "gitea.gitea_get_file",
+    "gitea.gitea_list_branches",
+    "gitea.gitea_open_pulls"
   ],
   "own-secrets": {
     "broker": "${dir:state}/broker"