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/services/design_cascade.py

@@ -97,6 +97,7 @@ class ResolvedToken:
     contributions: dict[str, tuple[Contribution, ...]]
     group_name: str | None
     purpose: str | None
+    rationale: str | None
     supersedes: tuple[str, ...]
     order_index: int
 
@@ -133,6 +134,7 @@ class ResolvedToken:
             "name": self.name,
             "group_name": self.group_name,
             "purpose": self.purpose,
+            "rationale": self.rationale,
             "supersedes": list(self.supersedes),
             "order_index": self.order_index,
             "value_by_mode": self.value_by_mode,
@@ -210,11 +212,11 @@ def resolve_tokens(
             meta = metadata.setdefault(
                 token.name,
                 {
-                    "group_name": None, "purpose": None,
+                    "group_name": None, "purpose": None, "rationale": None,
                     "supersedes": None, "order_index": None,
                 },
             )
-            for field in ("group_name", "purpose"):
+            for field in ("group_name", "purpose", "rationale"):
                 if meta[field] is None:
                     meta[field] = getattr(token, field, None)
             # order_index alone treats 0 as UNSTATED rather than "first",
@@ -240,6 +242,7 @@ def resolve_tokens(
             },
             group_name=metadata[name]["group_name"],
             purpose=metadata[name]["purpose"],
+            rationale=metadata[name]["rationale"],
             supersedes=metadata[name]["supersedes"] or (),
             order_index=metadata[name]["order_index"] or 0,
         )