Migrate goreleaser config off deprecated keys

`goreleaser check` against v2.15.4 flagged two deprecations:

* `archives.format: tar.gz` → `archives.formats: [tar.gz]`
  (renamed in v2.6; still accepts a string but plural is the
  forward-compatible shape).
* `brews:` → `homebrew_casks:` (renamed in v2.10). For a CLI
  binary the cask handles binary placement implicitly via
  `binary "atlas"`, so the `install: bin.install "atlas"` line
  was dropped.

Because atlas binaries aren't signed/notarized, the cask needs a
`hooks.post.install` step that strips the macOS quarantine
xattr — otherwise `brew install` succeeds but Gatekeeper refuses
to launch the binary. The hook is guarded by `OS.mac?` so it's a
no-op on Linuxbrew.

Verified with `goreleaser check` (exits 0) and `goreleaser
release --snapshot --clean --skip=publish` (four archives,
checksums match, generated cask validates against checksums.txt
and renders the expected per-platform url/sha256 + postflight
xattr block).

Author: sethdeckard

.goreleaser.yaml

@@ -16,22 +16,29 @@ builds:
       - arm64
 
 archives:
-  - format: tar.gz
+  - formats: [tar.gz]
     name_template: "{{ .ProjectName }}_{{ .Version }}_{{ .Os }}_{{ .Arch }}"
 
 checksum:
   name_template: "checksums.txt"
 
-brews:
+homebrew_casks:
   - repository:
       owner: sethdeckard
       name: homebrew-tap
       token: "{{ .Env.HOMEBREW_TAP_TOKEN }}"
     homepage: https://github.com/sethdeckard/atlas
     description: "Curated map of every Git repo under ~/projects"
     license: MIT
-    install: |
-      bin.install "atlas"
+    # atlas binaries aren't signed/notarized; strip macOS quarantine
+    # so `brew install` produces a runnable binary. Linux/Linuxbrew
+    # is a no-op via the OS.mac? guard.
+    hooks:
+      post:
+        install: |
+          if OS.mac?
+            system_command "/usr/bin/xattr", args: ["-dr", "com.apple.quarantine", "#{staged_path}/atlas"]
+          end
 
 changelog:
   sort: asc