fix(6855): the fence keeps its files where no compaction sweeps them, and builds id lists under mawk

The project a session entered now lives in grimoire-fence, which the
compaction clear never visits, and session end removes it. Arrays of ids
in a tool's input were sent as [,"3","4"] under mawk, which creates an
array element as soon as it is assigned to.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugin/hooks/grimoire_project_fence.sh

@@ -51,7 +51,9 @@ session_id=$(grimoire_json_pick "$event_flat" '.session_id')
 tool_name=$(grimoire_json_pick "$event_flat" '.tool_name')
 hook_event=$(grimoire_json_pick "$event_flat" '.hook_event_name')
 cwd=$(grimoire_json_pick "$event_flat" '.cwd')
-state_dir="${TMPDIR:-/tmp}/grimoire-priorart"
+# Its own directory, which no compaction sweeps: the project a session
+# entered stays its project for the whole session.
+state_dir="${TMPDIR:-/tmp}/grimoire-fence"
 safe_sid=$(printf '%s' "${session_id:-none}" | tr -c 'A-Za-z0-9._-' '_')
 entered_file="$state_dir/${safe_sid}.entered"
 
@@ -135,7 +137,11 @@ fence_tool_input() {
     }
     $2 ~ /^\.tool_input\.[A-Za-z_]+\[[0-9]+\]$/ && $3 != "null" && length($3) <= 200 {
       k = substr($2, 13); sub(/\[.*$/, "", k); add(k)
-      arr[k] = (k in arr) ? arr[k] ",\"" $3 "\"" : "\"" $3 "\""
+      # Tested before assigning: mawk creates arr[k] as soon as it is the
+      # target, so the test inside the assignment would always be true.
+      item = "\"" $3 "\""
+      if (k in arr) item = arr[k] "," item
+      arr[k] = item
     }
     END {
       printf "{"