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/services/design_systems.py

@@ -37,7 +37,6 @@ from scribe.services.design_cascade import (
     resolve_tokens,
     would_cycle,
 )
-from scribe.services.settings import get_setting
 
 logger = logging.getLogger(__name__)
 
@@ -135,40 +134,6 @@ async def get_design_system(user_id: int, design_system_id: int) -> DesignSystem
     return system
 
 
-# Which design system this install's own UI is built from. A plain setting
-# rather than a column: no migration, discoverable in the Settings UI (rule
-# #25), and honest about being a per-install claim rather than a property of the
-# system — the same system can be the record for an app that never loads it.
-UI_DESIGN_SYSTEM_SETTING = "ui_design_system_id"
-
-
-async def ui_design_system(user_id: int) -> tuple[int | None, DesignSystem | None]:
-    """The design system this install says its UI is built from.
-
-    Returns `(id, system)`. Three outcomes, deliberately distinguishable:
-
-    - `(None, None)` — nothing designated. The NORMAL state for any install but
-      the one that set it up (rule #115), not an error.
-    - `(id, None)` — designated, but gone or not readable by this caller. A
-      misconfiguration worth naming rather than silently degrading to "none",
-      which is exactly the failure that orphaned the panel this feeds (#2419).
-    - `(id, system)` — designated and readable.
-
-    A non-numeric setting value reads as nothing designated: the value is only
-    ever written by a `<select>` of real ids, so garbage here means hand-edited
-    or stale, and refusing to guess is better than raising on a page load.
-    """
-    raw = (await get_setting(user_id, UI_DESIGN_SYSTEM_SETTING, "")).strip()
-    if not raw:
-        return None, None
-    try:
-        system_id = int(raw)
-    except ValueError:
-        logger.warning("Ignoring non-numeric %s: %r", UI_DESIGN_SYSTEM_SETTING, raw)
-        return None, None
-    return system_id, await get_design_system(user_id, system_id)
-
-
 async def list_design_systems(user_id: int) -> list[DesignSystem]:
     """The caller's own systems, ordered by title. Empty is normal."""
     async with async_session() as session: