Simplify CI to only test on Ubuntu

The library only does YAML parsing and byte/string manipulation - nothing
platform-specific. Testing on Ubuntu across Python 3.10-3.13 is sufficient.

Reduces CI job count from 12 (3 OS × 4 Python) to 4 (1 OS × 4 Python).

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

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

Author: ryan-williams

.github/workflows/ci.yml

@@ -9,10 +9,9 @@ on:
 
 jobs:
   test:
-    runs-on: ${{ matrix.os }}
+    runs-on: ubuntu-latest
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
         python-version: ['3.10', '3.11', '3.12', '3.13']
     steps:
       - uses: actions/checkout@v4