chore: make Gitea the primary remote, GitHub as mirror

- Migrate CI workflow from GitHub Actions to Gitea Actions
- Update /pr command to create/merge PRs on Gitea only
- Update /release command to create releases on Gitea only
- Remove /pr-create and /pr-merge commands (consolidated into /pr)
- Push code and tags to both remotes for GitHub mirroring

.gitea/workflows/ci.yml

@@ -0,0 +1,26 @@
+name: CI
+
+on:
+  push:
+    branches:
+      - '**'
+  pull_request:
+    branches:
+      - '**'
+
+jobs:
+  build-test:
+    name: Build and Test
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: '1.25.4'
+          cache: true
+
+      - name: Run CI checks
+        run: bash ./ci-check.sh