ci(github): add GitHub Actions CI and release workflows for k3nnyfr/releaser mirror

- .github/workflows/ci.yml: same vet/staticcheck/test/build checks as Gitea CI
- .github/workflows/release.yml: same five binaries and build flags as the
  Gitea release workflow, published with gh release create
- docs: mention the GitHub mirror and both release pages; GitHub Actions
  example downloads from GitHub releases using RUNNER_OS/RUNNER_ARCH
- docs: fix broken latest/download URL, clone URL and Go version requirement

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -0,0 +1,36 @@
+name: ci
+
+on:
+  push:
+    branches:
+      - '**'
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  ci:
+    name: vet, staticcheck, test, build
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v5
+
+      - name: Set up Go
+        uses: actions/setup-go@v6
+        with:
+          go-version-file: go.mod
+
+      - name: vet
+        run: go vet ./...
+
+      - name: staticcheck
+        run: go tool staticcheck ./...
+
+      - name: test
+        run: go test ./...
+
+      - name: build
+        run: go build -o /dev/null ./cmd/...