feat(snippets): a snippet has notes; when_to_use is the situation it is ranked on (#4378)

A snippet had no field for prose, so what a session learned about one went
into when_to_use — the trigger joined onto every chunk it is embedded as. A
sweep found write-ups of up to 3 KB there, headings and all.

- notes: stored after the code under `## Notes`, parsed back from the body,
  carried by every path that rebuilds it (update, merge, un-merge). A
  snippet with no notes composes the body it always did.
- create/update_snippet (MCP) take notes and return trigger_advice when
  when_to_use is long, headed or multi-paragraph. Advice, not a refusal.
- Tool docs, the reusing-code skill and the editor hint describe the trigger
  as the situation and point the explanation at notes.
- Editor gains a Notes field; the detail view renders it as markdown.

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

plugin/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "scribe",
   "description": "Scribe for Claude Code: connects the scribe MCP server, adds the hooks that deliver live project state and relevant records at the right moment, ships the shared client-neutral Scribe skills (using-scribe, writing-plans, reporting-back, systematic-debugging, verification, brainstorming, reusing-code, shape-accounting), and syncs your saved Scribe Processes as skills (/scribe:sync).",
-  "version": "2026.09.23.2008",
+  "version": "2026.09.23.2127",
   "author": {
     "name": "Bryan Van Deusen"
   },