fix: Replace post_install with caveats for skill install

Homebrew's sandbox blocks post_install from writing to ~/.claude/.
Using caveats instead displays a message telling users to run
`mcs skill install` manually after installation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cv

.goreleaser.yml

@@ -52,8 +52,9 @@ brews:
     license: MIT
     install: |
       bin.install "mcs"
-    post_install: |
-      system "#{bin}/mcs", "skill", "install"
+    caveats: |
+      To enable Claude Code integration, run:
+        mcs skill install
     test: |
       system "#{bin}/mcs", "--version"