feat: TypeScript quality gates — ESLint, pre-commit hooks, Gitea CI

- ESLint config (eslint.config.ts) with typescript-eslint covering all TS source
- pnpm lint and pnpm typecheck scripts wired in workspace root package.json
- .githooks/pre-commit and .githooks/pre-push blocking on typecheck+lint failures
- Makefile install target activates hooks via git config core.hooksPath
- .gitea/workflows/ci.yml runs pnpm typecheck && pnpm lint on every PR to main
- AGENTS.md documents hook setup, lint/typecheck commands, and push workflow
- Fix TS2448/TS2454 in artifacts/mobile/app/onboarding.tsx (slideStyles used
  before declaration) so pnpm typecheck exits 0

Fixes #53

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

artifacts/mobile/app/onboarding.tsx

@@ -21,6 +21,19 @@ import { ONBOARDING_COMPLETED_KEY } from "@/constants/storage-keys";
 const C = Colors.dark;
 const { width: SCREEN_WIDTH } = Dimensions.get("window");
 
+const slideStyles = StyleSheet.create({
+  iconCircle: {
+    width: 140,
+    height: 140,
+    borderRadius: 70,
+    backgroundColor: C.surfaceElevated,
+    borderWidth: 1,
+    borderColor: C.border,
+    alignItems: "center",
+    justifyContent: "center",
+  },
+});
+
 type Slide = {
   id: string;
   icon: React.ReactNode;
@@ -158,19 +171,6 @@ export default function OnboardingScreen() {
   );
 }
 
-const slideStyles = StyleSheet.create({
-  iconCircle: {
-    width: 140,
-    height: 140,
-    borderRadius: 70,
-    backgroundColor: C.surfaceElevated,
-    borderWidth: 1,
-    borderColor: C.border,
-    alignItems: "center",
-    justifyContent: "center",
-  },
-});
-
 const styles = StyleSheet.create({
   container: {
     flex: 1,