chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/dependabot.yml

@@ -3,9 +3,5 @@ updates:
 - package-ecosystem: gomod
   directory: "/"
   schedule:
-    interval: daily
+    interval: weekly
   open-pull-requests-limit: 10
-  ignore:
-  - dependency-name: github.com/alecthomas/chroma
-    versions:
-    - 0.9.1