fix: Volcengine model list pollution + missing by-cwd route

- provider-catalog: getDefaultModelsForProvider() now returns preset's
  defaultModels even when empty, instead of falling through to
  ANTHROPIC_DEFAULT_MODELS. Fixes Volcengine (defaultModels: []) incorrectly
  showing Sonnet/Opus/Haiku.
- providers/models route: remove DEFAULT_MODELS fallback when both DB and
  catalog are empty — providers like Volcengine that require user-specified
  model names should show only those models.
- Add missing src/app/api/chat/sessions/by-cwd/route.ts that was untracked;
  without it worktree switching would 404 in release builds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/api/chat/sessions/by-cwd/route.ts

@@ -0,0 +1,15 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getLatestSessionByWorkingDirectory } from '@/lib/db';
+
+export async function GET(req: NextRequest) {
+  const cwd = req.nextUrl.searchParams.get('cwd');
+  if (!cwd) {
+    return NextResponse.json({ error: 'cwd is required' }, { status: 400 });
+  }
+
+  const session = getLatestSessionByWorkingDirectory(cwd);
+  if (session) {
+    return NextResponse.json({ sessionId: session.id });
+  }
+  return NextResponse.json({ sessionId: null });
+}

src/app/api/providers/models/route.ts

@@ -128,14 +128,14 @@ export async function GET() {
         capabilities: m.capabilities as Record<string, unknown> | undefined,
       }));
 
-      // Start with DB models + catalog defaults
+      // Start with DB models + catalog defaults.
+      // If both are empty (e.g. Volcengine where user must specify model names),
+      // leave rawModels empty — do NOT fall back to DEFAULT_MODELS (Sonnet/Opus/Haiku).
       if (dbModels.length > 0) {
         const dbIds = new Set(dbModels.map(m => m.value));
         rawModels = [...dbModels, ...catalogRaw.filter(m => !dbIds.has(m.value))];
-      } else if (catalogRaw.length > 0) {
-        rawModels = [...catalogRaw];
       } else {
-        rawModels = [...DEFAULT_MODELS];
+        rawModels = [...catalogRaw];
       }
 
       // Inject role_models_json.default into the list if not already present