Don't commit .bundle or vendor/bundle into update branches

\`git add -A\` was staging bundler files that \`ruby/setup-ruby\` writes
into the workspace (.bundle/config, vendor/bundle/). These got
committed into the first importmap branch. On the next checkout back
to origin/main, git deleted them — and with the bundle path config and
cached gems gone, subsequent \`bin/importmap pin\` calls failed with
"Could not find gems".

Stage only the paths \`bin/importmap pin\` is expected to modify:
\`config/importmap.rb\` and \`vendor/javascript/\` (for vendoring mode).

Author: neilvcarvalho

lib/git_client.rb

@@ -30,7 +30,7 @@ def checkout_fresh_branch(branch:, base:)
       # if a commit was actually created, false if there was nothing to
       # commit (which usually means `bin/importmap pin` was a no-op).
       def commit_all(message:)
-        @runner.run!("git", "add", "-A")
+        @runner.run!("git", "add", "config/importmap.rb", "vendor/javascript")
         # `git diff --cached --quiet` exits 0 if there are no staged changes.
         diff = @runner.run("git", "diff", "--cached", "--quiet")
         return false if diff.success?