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.

alembic/versions/0074_design_prose.py

@@ -0,0 +1,44 @@
+"""central prose on a design system, and rationale on a token
+
+Revision ID: 0074
+Revises: 0073
+Create Date: 2026-07-31
+
+The operator's shape for #254: prose doesn't live as one-offs, it lives centrally
+on the system. Two fields rather than one per category:
+
+  design_systems.guidance   the narrative a token table cannot hold — aesthetic,
+                            voice and tone, what is deliberately out of scope.
+                            Markdown, free-form.
+  design_tokens.rationale   WHY this 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.
+
+Free-form rather than a column per category on purpose. A schema with `voice`,
+`aesthetic` and `scope` columns would bake one rulebook's table of contents into
+every install (rule #115), and the next install's design system would have three
+empty columns and nowhere to put what it actually cares about.
+
+Both nullable: a design system with no prose at all is complete, not a draft.
+
+Downgrade drops both columns and the prose with them.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "0074"
+down_revision = "0073"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("design_systems", sa.Column("guidance", sa.Text(), nullable=True))
+    op.add_column("design_tokens", sa.Column("rationale", sa.Text(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("design_tokens", "rationale")
+    op.drop_column("design_systems", "guidance")