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/lexers/embedded/toml.xml

@@ -5,6 +5,7 @@
     <filename>*.toml</filename>
     <filename>Pipfile</filename>
     <filename>poetry.lock</filename>
+    <filename>uv.lock</filename>
     <mime_type>text/x-toml</mime_type>
   </config>
   <rules>