Add GitHub Actions CI workflow

Runs tests, type checking, build, and packages .vsix on push/PR to main.
Uploads .vsix as artifact.

Author: 7tg

.github/workflows/ci.yml

@@ -0,0 +1,32 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: oven-sh/setup-bun@v2
+        with:
+          bun-version: latest
+
+      - run: bun install --frozen-lockfile
+
+      - run: bun test
+
+      - run: bun run lint
+
+      - run: bun run build
+
+      - run: bun run vsce-package
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: plannotator-webview.vsix
+          path: "*.vsix"