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>

vendor/github.com/alecthomas/chroma/v2/styles/github-dark.xml

@@ -1,6 +1,6 @@
 <style name="github-dark">
   <entry type="Error" style="#f85149"/>
-  <entry type="LineHighlight" style="#6e7681"/>
+  <entry type="LineHighlight" style="bg:#6e7681"/>
   <entry type="LineNumbers" style="#6e7681"/>
   <entry type="Background" style="#e6edf3 bg:#0d1117"/>
   <entry type="Keyword" style="#ff7b72"/>
@@ -42,4 +42,4 @@
   <entry type="GenericTraceback" style="#ff7b72"/>
   <entry type="GenericUnderline" style="underline"/>
   <entry type="TextWhitespace" style="#6e7681"/>
-</style>
+</style>