fix: commit Gemfile.lock for GitHub Actions builds

GitHub Actions requires a committed Gemfile.lock to properly install
dependencies with bundler-cache. This fixes the build failure where
asciidoctor was not being installed.

Changes:
- Remove Gemfile.lock from .gitignore
- Add vendor/bundle/ to .gitignore (local gem installation)
- Commit generated Gemfile.lock

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

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

Author: davidjpeacock

.gitignore

@@ -2,8 +2,8 @@
 .asciidoctor/
 
 # Ruby
-Gemfile.lock
 .bundle/
+vendor/bundle/
 
 # Build output
 build/

Gemfile.lock

@@ -0,0 +1,16 @@
+GEM
+  remote: https://rubygems.org/
+  specs:
+    asciidoctor (2.0.26)
+    rouge (4.6.1)
+
+PLATFORMS
+  ruby
+  x86_64-linux-gnu
+
+DEPENDENCIES
+  asciidoctor (~> 2.0)
+  rouge (~> 4.1)
+
+BUNDLED WITH
+   2.6.7