fix(plugin): a subagent hand-back is not an operator prompt (#4785)

`<agent-message …>` turns were scored by auto-inject and the prompt rule
arm as if the operator had typed them, spending a retrieval and writing a
retrieval_logs row that reads as a real message (log 54355, found by the
#4772 review). Same class as #4142. Matched up to the tag name, since the
tag carries attributes; `<agent-messages>` and the tag named mid-sentence
are still retrieved against.

Plugin 2026.10.04.0125.

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

plugin/hooks/scribe_defs.sh

@@ -79,6 +79,7 @@ scribe_skip_prompt() {
   _scribe_prompt=${1#"${1%%[![:space:]]*}"}
   case "$_scribe_prompt" in
     "<task-notification>"*|\
+    "<agent-message>"*|"<agent-message "*|\
     "<command-name>"*|"<command-message>"*|"<command-args>"*|\
     "<local-command-stdout>"*|"<local-command-stderr>"*|"<local-command-caveat>"*)
       return 0 ;;