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

src/scribe/routes/design.py

@@ -1,45 +0,0 @@
-"""This install's UI surface — which design system it claims to be built from.
-
-Kept separate from the design-systems CRUD blueprint on purpose. That one is
-the RECORD: create a system, move a token, read the cascade. This one answers a
-question about the RUNNING APP, and it exists because those are not the same
-question. A design system can be a perfect record of a stylesheet the app never
-loaded.
-
-The client owns the other half. `utils/designTokens.ts` reads what the browser
-actually resolved, which is the one thing no server can report, and compares it
-to what this endpoint's system declares. So the comparison is
-"does the app agree with its own sheet?" rather than "is the record
-self-consistent?", which would be a tautology — the sheet is generated from the
-record (#2419).
-"""
-from quart import Blueprint, jsonify
-
-from scribe.auth import get_current_user_id, login_required
-from scribe.services import design_systems as ds_svc
-
-design_bp = Blueprint("design", __name__, url_prefix="/api/design")
-
-
-@design_bp.get("/ui-system")
-@login_required
-async def get_ui_system():
-    """The design system this install designated as the source of its own UI.
-
-    Returns `{"design_system_id": int|null, "title": str|null}`.
-
-    Both nulls is the NORMAL case, not an error — an install that has not
-    designated one has nothing to check the running app against, and the client
-    shows an explanatory empty state (rule #115).
-
-    An id with a null title is the third case and the reason the id is returned
-    separately: designated, but deleted or not readable by this caller. Folding
-    that into "none designated" is precisely how a feature comes to render a
-    reassuring empty state forever.
-    """
-    uid = get_current_user_id()
-    system_id, system = await ds_svc.ui_design_system(uid)
-    return jsonify({
-        "design_system_id": system_id,
-        "title": system.title if system else None,
-    })