feat(build): add GitHub mirror CI and release workflows

Mirror the repository to github.com/k3nnyfr/glint and build releases there
as well as on Gitea.

- .github/workflows/ci.yml: vet, staticcheck, test, build
- .github/workflows/release.yml: build the six platform binaries and
  publish a GitHub release on each v* tag
- action.yml: download from GitHub releases and pick the binary matching
  the runner OS/arch instead of always linux-amd64
- README/INSTALL/CHANGELOG: document the mirror, Homebrew tap renamed to
  k3nnyfr/homebrew-glint

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@v4
+
+      - name: Setup Go
+        uses: actions/setup-go@v5
+        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 ./cmd/glint/...