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>

src/scribe/routes/snippets.py

@@ -32,7 +32,10 @@ logger = logging.getLogger(__name__)
 snippets_bp = Blueprint("snippets", __name__, url_prefix="/api/snippets")
 
 # Fields the create/update payload may carry, mapped straight to the service.
-_STR_FIELDS = ("name", "code", "language", "signature", "when_to_use", "repo", "path", "symbol")
+_STR_FIELDS = (
+    "name", "code", "language", "signature", "when_to_use", "repo", "path", "symbol",
+    "notes",
+)
 
 
 async def _load_snippet(uid: int, snippet_id: int):
@@ -106,6 +109,7 @@ async def create_snippet_route():
                 path=data.get("path", ""),
                 symbol=data.get("symbol", ""),
                 locations=data.get("locations"),
+                notes=data.get("notes", ""),
             ),
             project_id=project_id,
             is_task=False,
@@ -139,6 +143,7 @@ async def create_snippet_route():
         locations=data.get("locations"),
         tags=data.get("tags"),
         project_id=project_id,
+        notes=data.get("notes", ""),
     )
     if data.get("system_ids") is not None:
         await systems_svc.set_record_systems(uid, note.id, data["system_ids"])