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/c#.xml

@@ -19,10 +19,10 @@
       <rule pattern="\\\n">
         <token type="Text"/>
       </rule>
-      <rule pattern="///[^\n\r]+">
+      <rule pattern="///[^\n\r]*">
         <token type="CommentSpecial"/>
       </rule>
-      <rule pattern="//[^\n\r]+">
+      <rule pattern="//[^\n\r]*">
         <token type="CommentSingle"/>
       </rule>
       <rule pattern="/[*].*?[*]/">