feat(build): add multi-platform release targets, Homebrew formula, and INSTALL.md

Taskfile:
- Add build-linux-arm64, build-darwin-amd64, build-darwin-arm64 targets
- Rename build-linux to build-linux-amd64 (keep build-linux alias)
- Rename Windows output to glint-<tag>-windows-amd64.exe for consistency
- Add build-release task that builds all five platforms in one shot

Formula/glint.rb:
- Homebrew source-build formula; depends_on "go" => :build
- tap: brew tap k3nny/glint https://github.com/k3nny/homebrew-glint
- Includes basic test block (--version + lint a trivial pipeline)

INSTALL.md:
- Pre-built binary download instructions for all five platforms
- Homebrew tap setup and formula update procedure
- go install one-liner
- Link to README integrations section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Formula/glint.rb

@@ -0,0 +1,29 @@
+class Glint < Formula
+  desc "Local linter and validator for .gitlab-ci.yml pipelines"
+  homepage "https://git.k3nny.fr/k3nny/glint"
+  url "https://git.k3nny.fr/k3nny/glint/archive/v0.3.1.tar.gz"
+  # Update sha256 on each release: sha256sum glint-vX.Y.Z.tar.gz
+  sha256 ""
+  license "Apache-2.0"
+  head "https://git.k3nny.fr/k3nny/glint.git", branch: "main"
+
+  depends_on "go" => :build
+
+  def install
+    system "go", "build",
+           "-ldflags", "-X main.version=#{version}",
+           "-o", bin/"glint",
+           "./cmd/glint/..."
+  end
+
+  test do
+    assert_match version.to_s, shell_output("#{bin}/glint --version")
+    (testpath/".gitlab-ci.yml").write <<~YAML
+      stages: [build]
+      build-job:
+        stage: build
+        script: echo ok
+    YAML
+    system bin/"glint", "check", ".gitlab-ci.yml"
+  end
+end