refactor(design): retire /design — a surface that could only inspect itself

The design surface is for the projects an install tracks. /design read the
running app's own stylesheet — names out of a bundled theme.css, values out of
getComputedStyle(document.documentElement) — so it could only ever describe the
instance serving the page. Scribe is one project among the projects Scribe
tracks; it gets no view hardcoded into every install.

The mechanism that makes this a tool rather than a mirror already existed and
already covers Scribe: scripts/check_design_tokens.py runs in CI against a
sheet path it knows nothing about, using check_code_against_tokens — the same
engine behind check_snippets_against_system. /design was redundant even here.

Removed: DesignView, DesignTabs (nothing left to tab between), api/design.ts,
routes/design.py and its blueprint, the /design route, ui_design_system() and
its setting, and the Settings picker that designated "this app's UI".

utils/designTokens.ts and utils/designDrift.ts go with it — between them they
were the browser-reading half. What survives is utils/designValues.ts, which
works on a record rather than a document: valueForMode, modesPresent, and
resolveDeclared.

resolveDeclared gained real isolation in the move. Custom properties inherit
and `all: initial` does not reset them, so a probe sitting in this page would
resolve any reference a record leaves undeclared against the SURROUNDING app's
tokens — previewing another project's system would quietly borrow this one's
palette wherever that system was incomplete, and a token already reported under
unknown_refs would render as though it were fine. Undeclared references are now
blanked on the probe first, so they resolve to nothing, which is what the record
says they are.

Migration 0075 absorbs ui_design_system_id alongside design_rulebook_id rather
than an 0076 undoing it: 0075 has not run anywhere, since dev is unmerged and
deploys come from main. Both keys named a design source for the running
install, and a project already carries its own pointer.

This retires the agreement panel shipped yesterday. It asked whether the sheet
was actually loaded and applied — the one question a record cannot answer about
itself — but only ever about the app you are already inside. Nothing replaces
it; recorded in #2430 rather than quietly dropped.

Step 1 of milestone #274.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

frontend/src/views/DesignSystemsView.vue

@@ -1,9 +1,13 @@
 <script setup lang="ts">
 /**
- * Design systems — editing the stylesheet Scribe holds (milestone #254 step 5).
+ * Design systems — the stylesheets this install RECORDS, for the projects it
+ * tracks (milestone #254 step 5).
  *
- * Sibling of /design, which shows the system as the BROWSER has it. This page
- * shows it as the RECORD has it, which is the half you can change.
+ * It had a sibling, `/design`, which showed the system as the BROWSER had it —
+ * names out of a bundled stylesheet, values out of `getComputedStyle`. That
+ * could only ever describe the install serving the page, so it was a mirror
+ * rather than a tool and was retired (#274). Everything here works on a system
+ * whose app this browser has never loaded.
  *
  * The layout follows the model rather than decorating it. A system with a
  * parent holds only what it changes, so this page has two lists and they are
@@ -39,7 +43,6 @@ import {
   type SnippetCheck,
   type StylesheetResult,
 } from "@/api/designSystems";
-import DesignTabs from "@/components/DesignTabs.vue";
 import { ApiError } from "@/api/client";
 import { useToastStore } from "@/stores/toast";
 import StarterRolePicker from "@/components/StarterRolePicker.vue";
@@ -496,7 +499,6 @@ function isColourish(value: string): boolean {
 
 <template>
   <div class="ds-view">
-    <DesignTabs />
 
     <header class="ds-header">
       <h1>Design systems</h1>