feat: respect $VULP_REPO env override for update-check; add CI workflow

The compiled binary's update-check tile hardcoded dualfroz/fxg with no
way to override it, unlike the install scripts which already honor
$VULP_REPO — a fork or a future repo rename needed a rebuild just to
fix the tile. Also add .github/workflows/ci.yml (build + clippy -D
warnings + test on push/PR): the only existing workflow was the
tag-triggered release build, so nothing gated PRs before merge.

.github/workflows/ci.yml

@@ -0,0 +1,30 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+jobs:
+  check:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: clippy
+
+      - uses: Swatinem/rust-cache@v2
+
+      - name: Build
+        run: cargo build --all-targets
+
+      - name: Clippy
+        run: cargo clippy --all-targets -- -D warnings
+
+      - name: Test
+        run: cargo test