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>

INSTALL.md

@@ -27,7 +27,11 @@ sudo mv glint /usr/local/bin/
 
 ## Option 2 — Download a pre-built binary
 
-Pre-built binaries are attached to each [release](https://git.k3nny.fr/k3nny/glint/releases).
+Pre-built binaries are attached to each release, on both
+[git.k3nny.fr](https://git.k3nny.fr/k3nny/glint/releases) and the
+[GitHub mirror](https://github.com/k3nnyfr/glint/releases). The examples below
+use git.k3nny.fr; replace the base URL with
+`https://github.com/k3nnyfr/glint/releases/download` to download from GitHub.
 
 | Platform | File |
 |----------|------|
@@ -84,14 +88,14 @@ glint --version
 
 ## Option 3 — Homebrew (macOS and Linux)
 
-A Homebrew tap is available at `k3nny/glint`.
+A Homebrew tap is available at `k3nnyfr/glint`.
 
 > **First-time setup:** create a GitHub repository named `homebrew-glint`
-> under your account and copy [`Formula/glint.rb`](Formula/glint.rb) into it.
+> under the `k3nnyfr` account and copy [`Formula/glint.rb`](Formula/glint.rb) into it.
 > Users then install via the tap as shown below.
 
 ```bash
-brew tap k3nny/glint https://github.com/k3nny/homebrew-glint
+brew tap k3nnyfr/glint https://github.com/k3nnyfr/homebrew-glint
 brew install glint
 ```