feat: platform AI names behind Sign in with OTM (v0.3.0)

- OTM SSO: /api/auth/otm-sso verifies the control-plane ticket (vendored
  verifier), in-memory jti claim, mints an HMAC cookie (src/lib/session.ts);
  /api/auth/session + /api/auth/signout. No accounts, no DB.
- /api/ai-names: server-side call to the platform's metered gateway
  (ANTHROPIC_BASE_URL + per-app gateway token), 401 without a session,
  ~20 req/h per OTM account. Prompt/parser shared with the BYO-key path
  via src/lib/ai-prompt.ts.
- Finder: 'Sign in with OTM for AI names' link → 'AI names' toggle when
  signed in; own key still overrides.

Pairs with platform 0.116.0 (needsAnthropic + needsAuthSecret on names).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XStQKxPfEjrTvWo83KFCxG

src/app/api/auth/session/route.ts

@@ -0,0 +1,19 @@
+// Who's signed in (if anyone) and whether platform AI is even wired up on this
+// deployment. The UI decides from this what to show next to the AI toggle.
+
+import { NextResponse } from "next/server";
+import type { NextRequest } from "next/server";
+import { readSession, platformAiEnabled } from "@/lib/session";
+
+export const dynamic = "force-dynamic";
+
+export async function GET(req: NextRequest) {
+  const s = readSession(req);
+  return NextResponse.json(
+    {
+      platformAi: platformAiEnabled(),
+      user: s ? { email: s.email, name: s.name ?? null } : null,
+    },
+    { headers: { "cache-control": "no-store" } },
+  );
+}