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/models/design_system.py

@@ -32,6 +32,11 @@ class DesignSystem(Base, TimestampMixin, SoftDeleteMixin):
     )
     title: Mapped[str] = mapped_column(Text)
     description: Mapped[str | None] = mapped_column(Text, nullable=True)
+    # The narrative a token table cannot hold: aesthetic, voice and tone, what
+    # is deliberately out of scope. Free-form markdown rather than a column per
+    # category — a schema with `voice`/`aesthetic`/`scope` columns would bake one
+    # rulebook's table of contents into every install.
+    guidance: Mapped[str | None] = mapped_column(Text, nullable=True)
     # SET NULL, not CASCADE: deleting a family system must not delete every app
     # system that inherited from it. Orphaning turns each child into a root that
     # still holds its own overrides — recoverable. A cascade would destroy data
@@ -49,6 +54,7 @@ class DesignSystem(Base, TimestampMixin, SoftDeleteMixin):
             "owner_user_id": self.owner_user_id,
             "title": self.title,
             "description": self.description or "",
+            "guidance": self.guidance or "",
             "parent_id": self.parent_id,
             "created_at": self.created_at.isoformat() if self.created_at else None,
             "updated_at": self.updated_at.isoformat() if self.updated_at else None,
@@ -109,6 +115,11 @@ class DesignToken(Base, TimestampMixin, SoftDeleteMixin):
     # vocabulary, and a whitelist would bake one install's kit into the schema.
     group_name: Mapped[str | None] = mapped_column(Text, nullable=True)
     purpose: Mapped[str | None] = mapped_column(Text, nullable=True)
+    # WHY this token is this value — a different question from `purpose`, which
+    # is 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.
+    rationale: Mapped[str | None] = mapped_column(Text, nullable=True)
     # Literal values this token should be used INSTEAD OF, e.g. ["#fff",
     # "#ffffff"] on a text-on-action token.
     #
@@ -137,6 +148,7 @@ class DesignToken(Base, TimestampMixin, SoftDeleteMixin):
             "value_by_mode": self.value_by_mode or {},
             "group_name": self.group_name,
             "purpose": self.purpose,
+            "rationale": self.rationale,
             "supersedes": self.supersedes or [],
             "order_index": self.order_index,
             "created_at": self.created_at.isoformat() if self.created_at else None,