feat(design-systems): central prose — guidance on the system, rationale on tokens

Last piece of the architecture in #2296. The operator: "the prose doesn't have
to live as one offs, there's a central system for managing it."

Two fields, both free-form:

  design_systems.guidance   the narrative a token table cannot hold — aesthetic,
                            voice and tone, what is deliberately out of scope.
  design_tokens.rationale   WHY a token is this value, which is a different
                            question from `purpose` (what it is FOR). "Success
                            equals Moss, aligned by design" is a rationale;
                            "page bg, deepest surface" is a purpose. Rules carry
                            the first routinely and a token row had nowhere to
                            put it.

Free-form rather than a column per category, deliberately. A schema with
`voice`, `aesthetic` and `scope` columns would bake one rulebook's table of
contents into every install (rule #115), leaving the next install three empty
columns and nowhere for what it actually cares about. Both nullable: a design
system with no prose at all is complete, not a draft.

`rationale` cascades like `purpose` — deepest non-empty wins — so an app
overriding a colour keeps the family's reasoning rather than blanking it. Same
argument as `supersedes`: the override was about the value, not the meaning.

In the generated sheet the inline comment prefers `purpose` and falls back to
`rationale`, so a token carrying only the why still says something instead of
rendering bare.

src/scribe/routes/design_systems.py

@@ -54,6 +54,7 @@ async def create_design_system():
         user_id=_uid(),
         title=title,
         description=data.get("description") or None,
+        guidance=data.get("guidance") or None,
         parent_id=data.get("parent_id"),
     )
     if system is None:
@@ -74,7 +75,9 @@ async def get_design_system(design_system_id: int):
 @login_required
 async def update_design_system(design_system_id: int):
     data = await request.get_json() or {}
-    fields = {k: v for k, v in data.items() if k in ("title", "description")}
+    fields = {
+        k: v for k, v in data.items() if k in ("title", "description", "guidance")
+    }
     # Presence, not truthiness: `{"parent_id": null}` means "make this a root",
     # which a `if data.get("parent_id")` filter would silently drop.
     if "parent_id" in data:
@@ -203,6 +206,7 @@ async def create_design_token(design_system_id: int):
         value_by_mode=data.get("value_by_mode"),
         group_name=data.get("group_name") or None,
         purpose=data.get("purpose") or None,
+        rationale=data.get("rationale") or None,
         supersedes=data.get("supersedes"),
         order_index=data.get("order_index") or 0,
     )
@@ -218,8 +222,8 @@ async def update_design_token(token_id: int):
     fields = {
         k: v for k, v in data.items()
         if k in (
-            "name", "value_by_mode", "group_name", "purpose", "supersedes",
-            "order_index",
+            "name", "value_by_mode", "group_name", "purpose", "rationale",
+            "supersedes", "order_index",
         )
     }
     token = await ds_svc.update_token(_uid(), token_id, **fields)