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>

README.md

@@ -6,7 +6,7 @@
 
 A CI-friendly release automation tool for GitFlow workflows using Conventional Commits.
 
-**[Documentation](https://releaser.k3nny.fr)** · **[Repository](https://git.k3nny.fr/k3nny/releaser)**
+**[Documentation](https://releaser.k3nny.fr)** · **[Repository](https://git.k3nny.fr/k3nny/releaser)** · **[GitHub mirror](https://github.com/k3nnyfr/releaser)**
 
 ## Problem
 
@@ -42,6 +42,21 @@ By default, all releasable commits bump the **patch** component (minor is pinned
 | `chore:`, `docs:`, etc. | none   | —                                       |
 | unparseable msg  | none    | non-strict mode: silently ignored              |
 
+## Installation
+
+Pre-built binaries (`linux-amd64`, `linux-arm64`, `darwin-amd64`, `darwin-arm64`, `windows-amd64.exe`) are published for every tag on both forges:
+
+- **[git.k3nny.fr releases](https://git.k3nny.fr/k3nny/releaser/releases)** — primary repository
+- **[GitHub releases](https://github.com/k3nnyfr/releaser/releases)** — read-only mirror of git.k3nny.fr; issues and contributions go to the primary repository
+
+```bash
+curl -sSL https://github.com/k3nnyfr/releaser/releases/download/v1.10.0/releaser-v1.10.0-linux-amd64 \
+  -o /usr/local/bin/releaser
+chmod +x /usr/local/bin/releaser
+```
+
+See the [installation docs](https://releaser.k3nny.fr/installation/) for Docker and building from source.
+
 ## Usage
 
 ```bash