Add *.gem to .gitignore

`gem build mcp.gemspec` outputs the built gem into the working directory (e.g. mcp-0.7.1.gem)
rather than pkg/, which is already ignored.
Without this entry, the file shows up as an untracked file and risks being accidentally
staged and committed during release preparation or local development.

Author: koic

.gitignore

@@ -1,4 +1,5 @@
 .ruby-version
+/*.gem
 /.bundle/
 /.yardoc
 /_yardoc/