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.

frontend/src/views/DesignSystemsView.vue

@@ -179,6 +179,7 @@ async function submitCreate() {
 
 const editTitle = ref("");
 const editDescription = ref("");
+const editGuidance = ref("");
 const editParentId = ref<number | null>(null);
 const savingSystem = ref(false);
 
@@ -187,6 +188,7 @@ watch(
   (system) => {
     editTitle.value = system?.title ?? "";
     editDescription.value = system?.description ?? "";
+    editGuidance.value = system?.guidance ?? "";
     editParentId.value = system?.parent_id ?? null;
   },
   { immediate: true },
@@ -198,6 +200,7 @@ const systemDirty = computed(() => {
   return (
     editTitle.value !== system.title ||
     editDescription.value !== (system.description ?? "") ||
+    editGuidance.value !== (system.guidance ?? "") ||
     editParentId.value !== system.parent_id
   );
 });
@@ -212,6 +215,7 @@ async function saveSystem() {
     await updateDesignSystem(system.id, {
       title: editTitle.value.trim(),
       description: editDescription.value.trim(),
+      guidance: editGuidance.value.trim(),
       parent_id: editParentId.value,
     });
     await loadSystems();
@@ -263,6 +267,7 @@ function resetTokenForm() {
   tokenName.value = "";
   tokenGroup.value = "";
   tokenPurpose.value = "";
+  tokenRationale.value = "";
   tokenModes.value = [{ mode: "base", value: "" }];
   tokenSupersedes.value = "";
   editingTokenId.value = null;
@@ -274,6 +279,7 @@ function startEditToken(token: DesignToken) {
   tokenName.value = token.name;
   tokenGroup.value = token.group_name ?? "";
   tokenPurpose.value = token.purpose ?? "";
+  tokenRationale.value = token.rationale ?? "";
   tokenSupersedes.value = (token.supersedes ?? []).join(", ");
   const entries = Object.entries(token.value_by_mode);
   tokenModes.value = entries.length
@@ -301,6 +307,7 @@ async function submitToken() {
       value_by_mode: collectModes(),
       group_name: tokenGroup.value.trim() || null,
       purpose: tokenPurpose.value.trim() || null,
+      rationale: tokenRationale.value.trim() || null,
       supersedes: tokenSupersedes.value
         .split(",")
         .map((v) => v.trim())
@@ -629,6 +636,17 @@ function isColourish(value: string): boolean {
               <label class="field-label" for="edit-desc">Description</label>
               <input id="edit-desc" v-model="editDescription" class="input" type="text" />
             </div>
+            <div class="field">
+              <label class="field-label" for="edit-guidance">Guidance</label>
+              <textarea
+                id="edit-guidance" v-model="editGuidance" class="input" rows="5"
+                placeholder="Aesthetic, voice and tone, what's deliberately out of scope…"
+              ></textarea>
+              <p class="field-hint">
+                The prose a token table can't hold. Kept here rather than
+                scattered, so the system carries its own reasoning.
+              </p>
+            </div>
             <div class="field">
               <label class="field-label" for="edit-parent">Inherits from</label>
               <select id="edit-parent" v-model="editParentId" class="input">
@@ -918,6 +936,18 @@ function isColourish(value: string): boolean {
                 </div>
               </div>
 
+              <div class="field">
+                <label class="field-label" for="token-rationale">Why this value</label>
+                <input
+                  id="token-rationale" v-model="tokenRationale" class="input" type="text"
+                  placeholder="Success equals Moss, aligned by design"
+                />
+                <p class="field-hint">
+                  Distinct from purpose: purpose is what the token is FOR, this is
+                  why it is this value.
+                </p>
+              </div>
+
               <div class="field">
                 <label class="field-label" for="token-supersedes">Use instead of</label>
                 <input
@@ -1029,6 +1059,7 @@ function isColourish(value: string): boolean {
                     <div class="resolved-head">
                       <code class="token-name">{{ token.name }}</code>
                       <span v-if="token.purpose" class="token-meta">{{ token.purpose }}</span>
+                      <span v-if="token.rationale" class="supersedes">{{ token.rationale }}</span>
                       <span v-if="token.supersedes.length" class="supersedes">
                         instead of {{ token.supersedes.join(", ") }}
                       </span>
@@ -1286,6 +1317,12 @@ function isColourish(value: string): boolean {
 }
 
 
+textarea.input {
+  resize: vertical;
+  min-height: 5rem;
+  line-height: 1.5;
+}
+
 .mono {
   font-family: 'JetBrains Mono', ui-monospace, Menlo, Consolas, monospace;
 }