ci: add GitHub Actions build workflow

Compiles on macos-latest with xcodegen + xcodebuild (simulator, no signing).
Triggers on push/PR to main.

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

Author: vveerrgg

.github/workflows/ci.yml

@@ -0,0 +1,25 @@
+name: CI
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+jobs:
+  build:
+    runs-on: macos-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install xcodegen
+        run: brew install xcodegen
+      - name: Generate Xcode project
+        run: xcodegen generate
+      - name: Build
+        run: |
+          xcodebuild build \
+            -project NostrKey.xcodeproj \
+            -scheme NostrKey \
+            -configuration Debug \
+            -destination 'platform=iOS Simulator,name=iPhone 16,OS=latest' \
+            CODE_SIGN_IDENTITY="" \
+            CODE_SIGNING_REQUIRED=NO \
+            CODE_SIGNING_ALLOWED=NO