Add Windows to CI matrix (#85) (#86)

Adds windows-latest to the test job matrix alongside ubuntu and macos.
Two small supporting changes:

- fail-fast: false so a Windows-only failure doesn't kill the unix runs
  while we shake out platform-specific quirks.
- defaults.run.shell: bash so make build and the bash-driven smoke test
  run consistently across all three OSes (Git Bash provides bash on
  windows-latest by default).

Verified locally on Windows: make build produces a runnable binary
named "skern", and bash scripts/smoke_test.sh ./skern passes 57/57.
The smoke test already has a USERPROFILE/HOME bridge for Git Bash
(scripts/smoke_test.sh:130) so no script changes are needed.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: devrimcavusoglu

.github/workflows/ci.yml

@@ -9,9 +9,13 @@ on:
 jobs:
   test:
     strategy:
+      fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.os }}
+    defaults:
+      run:
+        shell: bash
     steps:
       - uses: actions/checkout@v4