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>

frontend/src/api/snippets.ts

@@ -30,6 +30,9 @@ export interface SnippetFields {
    *  no `locations`/`tags` predates that attribution and cannot be un-merged. */
   merged_from: { id: number; locations?: SnippetLocation[]; tags?: string[] }[];
   code: string;
+  /** Free text that is not the situation — why, history, caveats (#4378).
+   *  Kept out of `when_to_use`, which the snippet is ranked on. */
+  notes: string;
 }
 
 /** A full snippet record: the note dict plus the parsed `snippet` sub-object,
@@ -110,6 +113,7 @@ export interface SnippetInput {
   language?: string;
   signature?: string;
   when_to_use?: string;
+  notes?: string;
   locations?: SnippetLocation[];
   tags?: string[];
   project_id?: number | null;